During the 2009-2011 influenza seasons, 10.26% of the specimens isolated from patients in South Korea were subtyped as H3N2 viruses. Some oseltamivir-sensitive H3N2 samples exhibited different plaque morphologies, and were found to have novel mutations in the neuraminidase gene. In a subsequent analysis using NA mutant viruses, viral compensation against oseltamivir treatment was observed only in the N2 mutant virus. All things considered, these novel mutations may account for the exclusive characteristics of selected H3N2 viruses observed in plaque reduction assays.