The syncytium inducing (SI)/non-syncytium inducing (NSI) phenotype of various HIV-1 subtype isolates and their third variable (V3) domain amino acid sequences of env gene were analyzed. Eight out of 27 HIV-1 isolates (subtype A, B, E, D, and G) showed SI phenotype and 15 isolates (subtype A, B, and C) showed NSI phenotype, but we could not isolate viruses from 4 HIV-1 infected individuals (subtype A, B, and E). The CD4+ T cell counts showed significant difference between SI and NSI phenotype (p=0.03). It was found that there were positively charged amino acid residues (arginine or lysine) at either position 11 or position 25 in V3 loop of SI isolates. Also the insertion of positively charged amino acid residues between position 11 and the tetrameric motif sequence at the V3 loop was shown in HIV-1 isolate with SI phenotype. The NSI isolates had negatively charged or uncharged amino acid at both position 11 and 25 in the V3 loop. But HIV-1 subtype G was an exception in these characteristics. These results suggest that the syncytium inducing capacities of HIV-1 isolates be correlated with the presence of specific amino acids in the V3 sequence. It is needed further to identify other regions of HIV-1 subtype G correlated with viral phenotype.