en Epidemiology Epidemiology Original Research Original Research <strong>Background: </strong>Estimation of the basic reproduction number of an infectious disease is an important issue for controlling the infection. Here, we aimed to estimate the basic reproduction number (𝑅0) of COVID-19 in Iran.<br> <strong>&nbsp;&nbsp; Methods:</strong> To estimate 𝑅0 in Iran and Tehran, the capital, we used 3 different methods: exponential growth rate, maximum likelihood, and Bayesian time-dependent. Daily number of confirmed cases and serial intervals with a mean of 4.27 days and a standard deviation of 3.44 days with gamma distribution were used. Sensitivity analysis was performed to show the importance of generation time in estimating 𝑅₀.<br> <strong>&nbsp;&nbsp; Results:</strong> The epidemic was in its exponential growth 11 days after the beginning of the epidemic (Feb 19, 2020) with doubling time of 1.74 (CI: 1.58-1.93) days in Iran and 1.83 (CI: 1.39-2.71) in Tehran. Nationwide, the value of 𝑅₀ from February 19 to 29 using exponential growth method, maximum likelihood, and Bayesian time-dependent methods was 4.70 (95% CI: 4.23-5.23), 3.90 (95% CI: 3.47- 4.36), and 3.23 (95% CI: 2.94-3.51), respectively. In addition, in Tehran, 𝑅₀was 5.14 (95% CI: 4.15-6.37), 4.20 (95% CI: 3.38-5.14), and 3.94 (95% CI: 3.45-4.40) for exponential growth, maximum likelihood, and Bayesian time-dependent methods, respectively. Bayesian time dependent methods usually provide less biased estimates. The results of sensitivity analyses demonstrated that changes in the mean generation time affect estimates of 𝑅₀.<br> <strong>&nbsp;&nbsp; Conclusion: </strong>The estimate of 𝑅₀ for the COVID-19 ranged from 3.94 to 5.14 in Tehran and from 3.23 to 4.70 in nationwide using different methods, which were significantly larger than 1, indicating the potential of COVID-19 to cause an outbreak. COVID-19, Basic reproduction number, Transmissibility, Iran 668 675 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-5902-1&slc_lang=en&sid=1 Seyyedeh Sara Azimi saraazimi_2039@yahoo.com 200319475328460064234 200319475328460064234 No Student Research Committee, Department of Epidemiology, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran Fatemeh koohi fkoohi92@gmail.com 200319475328460064235 200319475328460064235 No Student Research Committee, Department of Epidemiology, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran Mohammad Aghaali dr.aghaali@yahoo.com 200319475328460064236 200319475328460064236 No Department of Epidemiology, School of Health, Qom University of Medical Sciences, Qom, & Department of Epidemiology, School of Public Health and Safety, Shahid Beheshti Uni-versity of Medical Sciences, Tehran, Iran Roya Nikbakht رویا نیکبخت roya.nikbakht2011@gmail.com 200319475328460064237 200319475328460064237 No Department of Biostatistics, Faculty of Health, Mazandaran University of Medical Sciences, Sari, Iran Maryam Mahdavi maryam.mahdavi1368@gmail.com 200319475328460064238 200319475328460064238 No Obesity Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran Yaser Mokhayeri yasermokhayeri@yahoo.com 200319475328460064239 200319475328460064239 No 6. Department of Epidemiology and Biostatistics, School of Public Health and Nutrition, Lorestan University of Medical Sciences, Khorramabad, Iran Rasool Mohammadi rasool.mehr2002@gmail.com 200319475328460064240 200319475328460064240 No 6. Department of Epidemiology and Biostatistics, School of Public Health and Nutrition, Lorestan University of Medical Sciences, Khorramabad, Iran Niloufar Taherpour taherpour.n@gmail.com 200319475328460064241 200319475328460064241 No Department of Epidemiology, School of Public Health and Safety, Shahid Beheshti Uni-versity of Medical Sciences, Tehran, Iran Mehran Nakhaeizadeh mehran.nakheai@yahoo.com 200319475328460064242 200319475328460064242 No 7. Modeling in Health Research Center, Institute for Futures Studies in Health, Kerman University of Medical Sciences, Kerman, Iran Davood Khalili ndavood@yahoo.com 200319475328460064243 200319475328460064243 No Prevention of Metabolic Disorders Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran Hamid Sharifi sharifihami@gmail.com 200319475328460064244 200319475328460064244 No HIV/STI Surveillance Research Center, and WHO Collaborating Center for HIV Surveil-lance, Institute for Futures Studies in Health, Kerman University of Medical Sciences, Kerman, Iran Seyed Saeed Hashemi Nazari saeedh_1999@sbmu.ac.ir 200319475328460064245 200319475328460064245 Yes Prevention of Cardiovascular Disease Research Center, Department of Epidemiology, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran