Classical swine fever (CSF) causes a highly contagious disease that affects domestic pig and wild boars and has serious economic implications. The envelope glycoprotein E2 of classical swine fever virus (CSFV) plays important roles in the viral replication, host interaction and pathogenesis. This study aimed to investigate positive selection pressure acting on E2 gene of CSF viruses by analyzing the E2 sequence of CSFV from field outbreak in South Korea during 1987-2017.The E2 gene of 18 CSFV isolates from South Korea during 1987-2017 was sequenced and analyzed. Phylogenetic analysis of E2 complete gene revealed that 11 strains during 2002-2017 were clustered in genotype 2.1b and 7 strains during 1987-1999 in genotype 3.2. In addition, the positive selection pressure acting on E2 gene of CSFV were investigated and six codons in genotype 2.1b and four codons in genotype 3.2 were found under positive selection on E2 complete gene. In this study, the genotype 2.1b viruses become predominant in Korea since 2002, suggesting possible field CSFV virus switching occurred from genotype 3 to rather exotic genotype 2 in South Korea. Moreover, the genotype 2.1b viruses have increased in mutation rate showingnew six codons under positive selection on E2 complete gene.