The aim of this study was to investigate whether loneliness and personality traits correlate with the treatment outcome of methamphetamine use disorder. In this 1-year longitudinal study, a total 106 participants (98 males, 8 females), with a mean age 36.3 ± 9.6 years were enrolled. We measured UCLA Loneliness Scale and Tridimensional Personality Questionnaire at baseline, while craving level at baseline, week 12, 24, 36, and 48. Urinary methamphetamine tests were given 17 times. For the evaluation of the data, multiple linear regression and generalized linear mixed models were used. The baseline results showed lower levels of the harm avoidance trait and higher levels of loneliness were significantly associated with higher craving levels (p=0.04 and 0.04). Moreover, loneliness was not only positively associated with craving levels (B=0.05, p<0.01) but with urinary methamphetamine positive results (B= 0.08, p=0.03) during one-year treatment. The findings suggested that loneliness was associated with poor methamphetamine treatment outcome (greater craving levels and higher proportion of positive methamphetamine urine tests) and lower harm avoidance traits are associated with higher craving levels.