<jats:sec id="sec001"> <jats:title>Objective</jats:title> <jats:p>This study aimed to estimate the effectiveness of a standard clinical training program for new graduate nurses in Vietnam.</jats:p> </jats:sec> <jats:sec id="sec002"> <jats:title>Methods</jats:title> <jats:p>A quasi-experimental longitudinal study with a difference-in-differences design was conducted. A total of 280 new graduate nurses completed a self-administered questionnaire. The intervention group consisted of 206 respondents (those having participated in standard clinical training) and the control group (those that did not receive training) of 74. Differences in mean increases in competency scores between the intervention and control groups were estimated. The effect size of the intervention was estimated by calculating Cohen’s d. A generalized linear model was employed to identify the factors associated with mean increases.</jats:p> </jats:sec> <jats:sec id="sec003"> <jats:title>Results</jats:title> <jats:p>The mean increase in total competency scores (range: 0–6 points) in the intervention group was 0.73 points greater than in the control group with an intermediate effect size (Cohen’s d = 0.53; 95% CI 0.26 to 0.80). A greater reduction in standard deviation of total competency scores in the intervention group was confirmed. Participation in standard clinical training produced a positive association with a mean increase in total competency score without significance (β = 0.04, P = 0.321). Provincial hospitals as clinical training venues had a significantly positive association (β = 0.11, P = 0.007) with the mean increase in total competency scores. Competency at pre-clinical training was negatively (β = -0.75, P < 0.001) associated with the mean increase.</jats:p> </jats:sec> <jats:sec id="sec004"> <jats:title>Conclusion</jats:title> <jats:p>Findings implied that the standard clinical training program could contribute to both increasing and standardizing new graduate nurses’ competencies in Vietnam. Further studies are needed to more precisely examine the attribution of standard clinical training to better increase new graduate nurses’ competencies.</jats:p> </jats:sec>