Although there is a report of a high rate of hepatitis B virus (HBV) infection in Mongolia, the entire nucleotide sequence of HBV circulating among Mongolian patients has not been reported. To obtain the complete nucleotide sequence of the Mongolian HBV, viral DNA was extracted from sera of patients with HBV infection. Six Mongolian HBV strains were amplified by PCR. Complete genomic sequences were determined for two Mongolian HBV isolates, MBT181 and MMU36. The entire genome of Mongolian HBV isolates was 3,182 bp long and genetic distance between Mongolian HBV isolates was 3.2%. Precore stop codon resulting from a guanine to adenine mutation at nucleotide 1,896 was detected in MBT181 strain. Based on phylogenetic analysis, the six Mongolian isolates were classified as genotype D.