Background: Nowadays, two kinds of drug are approved for treating Hepatitis B (HBV), such as Interferon and nucleostide analogs. The nucleos(t)ide analogs inhibit effectivly the replication of virus but easely emerge drug resistant mutants when long term using. Objectives: To determine the rates of drug resistant mutants in chronic HBV patients treated by nucleos(t)ide analogs more than 12 months. Methods: The cross-sectional study was done in the out-patient department at Tropical Diseases Hospital from July 2010 to October 2010. The PCR-based sequencing technique were applied to detect the resistant mutations in the polymerase gene. These patients must adhere to medication more than 1 year and they are defined as antiviral drug resistance when presenting one of the situations such as primary non-response (decreasing of serum HBV DNA 2 log10 IU/mL after at least 24 weeks of treatment) or virologic breakthrough or biochemical breakthrough. Results : 60 cases were enrolled in our study. 20/60 (33.3 percent) cases had at least one mutation. 19/58 (33.3 percent) case were Lamivudin resistant mutant, 1/28 ( 6 percent) case was Adefovir resistant mutant. None of Entecavir or Tenofovir resistant mutation was founded. Mutation rates of Lamivudin were 16 percent, 43 percent, 83 percent in 1, 2, or = 3 years, respectively. Mutation rates were significantly lower in group treated by the combination of Lamivudin and Adefovir (8.3 percent) than that in goup treated by Lamivudin alone (41 percent), p value = 0.03. Drugs-resistant mutations increased the rates of virologic breakthrough to 7.9 times and biochemical breakthrough to 4.2 times. No relation between HBV genotype or HBeAg statns and nucleos(t)ide analogs resistant mutations was found. Conclusion : Mutation rate is rather high in patients treated by nucleos(t)ide analogs, especially Lamivudin, and these mutations increase anually.