Abstract: In June 2009, the World Health Organization (WHO) confirmed that the novel influenza A, H1N1 as a pandemic. After six months, as of December 29, 2009, it was reported by WHO that more than 208 countries and territories were affected by the pandemic accounting for about 150,000 infected cases and at least 11,516 deaths. .Effective reproductive number is an index which considers the proportion of susceptible people in a community. There are different methods for calculation of basic reproductive number. The aim of this study was to evaluate the reproductive number using data from H1N1 patients in Tamilnadu.