The pandemic H1N1 strain of Influenza A virus [A(H1N1)pdm09] is now well adapted in human populations. However, it is still causing sporadic outbreaks worldwide with different severity. The present study was planned to understand the genetic diversity (based on the HA1 gene) of influenza A(H1N1)pdm09 strains circulating during the post pandemic period. The HA1 gene was selected because the HA1 protein is immunogenic, functions as a receptor binding site and indirectly affects transmission and pathogenicity of virus. A total of 2,818 cases were enrolled. Nasal/throat swabs from all cases were tested by one-step real time PCR for detection of influenza virus types and subtypes according to the CDC protocol. Of these, 134 cases were A(H1N1)pdm09 positive, 34 of which were screened for HA1 gene (position 434-905) sequencing (Big-Dye terminator using 3130 ABI, Genetic analyzer). Molecular and phylogenetic analysis was performed using PhyML approach (v. 3.0). All A(H1N1)pdm09 positive and negative cases were clinically characterized. Phylogentically, all Lucknow strains (n = 33) except one fall with the clade seven reference strain. One strain showed 99.9% similarities with clade one reference strain A/California/07/2009. In mutational analysis, 33 strains had the S220T mutation, which is at an antigenic site and characteristic of clade seven along with few minor mutations; K180I/T/Q, V190I, S200P, S202T, A203T, A214T, S220T, V251I, and A273T. These results suggest that clade seven was the most widely circulating clade in Lucknow and A(H1N1)pdm09 cases showed mild clinical symptoms as compared to A(H3N2) or influenza B cases.