Introduction: This study aimed to evaluate the impact of the helmet law on the changes in potential years of life lost (PYLL) due to traffic mortality and to examine modification effects of socioeconomic factors on the impacts in Vietnam., Methods: We applied an interrupted time series design using the Bayesian framework to estimate the impact of the law at the provincial level. Then, we used random effects meta-analysis to estimate the impact of the law at the country level and to examine the modification effects of socioeconomic factors., Results: The results indicate that the impacts varied among the provinces. These impacts could be classified by four main groups comprising positive impact, and positive impact without sustainability, possible positive impact, negative or inconsistent impact. For the country-level impact, the results reveal a significantly consistent change in monthly PYLLs at the level of 18 per 100 000 persons, and the post-trend was stable without significant change. The results of meta-regression show that 1 unit increase in the population density (persons/km 2 ), migration rate (%) and income (×1000 dong) are non-significantly associated with increases of PYLLs at 1.3, 27 and 27 per 100 000 person-months, respectively, whereas 1% increase in literacy associated with a decrease of PYLL at 44 per 100 000 person-months., Discussion: Further studies should be warranted to provide a comprehensive evaluation of the law implementation, including its acceptability, adoption, appropriateness, feasibility, cost-effectiveness and sustainability., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2020. No commercial re-use. See rights and permissions. Published by BMJ.)