HIV was classified into 3 groups: group M, N and O. Group M is an important group for epidemiolody. In Thailand, CRF01_AE and B are the most common subtype. Because of heterogeneity, HIV can recombine genes to another subtype and cause a new subtype. Forty-two plasma samples from HIV-infected patients were collected at Siriraj Hospital. Specific region of p17 (gag) and C2-V4 (env) genes of HIV were amplified by nested RT-PCR. PCR products were sequenced. Phylogenetic tree was constructed for epidemiological study. Results showed that phylogenetic analysis on env gene was found 100% of HIV subtype CRF01_AE (42/42). Whereas on gag (p17) gene, 69% and 31% were found in CRF01_AE and CRF15_01B subtype, respectively.