This cross-sectional study was conducted to identify the quality of life (QoL) and its related factors in methadone maintenance treatment (MMT) patients at four health facilities in Long An province, Vietnam in 2019. We interviewed 373 MMT patients using the WHOQOL-BREF measurement questionnaire and used their health records to collect some other data. The findings showed that the patients had a mean QoL score of 66.1 points. Factors associated with a higher QoL score included having employment, receiving support from relatives, and having health insurance. Therefore, it is recommended that patients should be assisted to participate in health insurance and supported by their relatives.