Recent 2009 flu pandemic is a global outbreak of a new strain of influenza A virus subtype H1N1. The H1N1 virus has crossed species barrier to human and apparently acquired the capability to transmit this disease from human to human. The NP is a multifunctional protein that not only encapsidates viral RNA (vRNA), but also forms homo-oligomer and thereby maintains RNP structure. It is also thought to be the key adaptor for virus and host cell interaction. Thus, it is one of the factor that play a key role in the pathogenesis of influenza A virus infection. Therefore, to understand the cause of pathogenicity of H1N1 virus, we have studied the structure-function relationship of different domains of NP. Our results showed that conservative mutation in NP of various strains were pathogenic in nature. However, non-conservative mutation slightly abrogated oligomerization and was therefore less pathogenic. Our results also suggest that beside tail and body domain, head domain may also participate in an oligomerization process.