Background:
In 2012, Vietnam National Assembly approved the first ever Law on Prevention and Control of Tobacco Harms. Enacted by the Law, since May 2013, pictorial health warnings (PHWs) covered 50% of the cigarette pack was obliged to print on all cigarette products. This study aims to assess the impact of PHWs on smokers' knowledge of smoking harms.

Methods:
Two rounds of a cross-sectional study were conducted in 6 provinces of Vietnam. The 1^st round was in May 2014 which was before the deadline for manufacturers to print PHWs (July 2014). The 2^nd round was in May 2015. Around 1,500 male smokers aged 18 to 35 were interviewed in each round. Respondents were asked to give their level of agreement towards 6 statements described the health consequences of smoking (same as the messages in PHWs). Based on their responses, the knowledge score was calculated.

Results:
Smokers' average knowledge score on smoking harms in round 2 (M=22.5, SD=3.7) was 0.86 point higher than in round 1 of the study (M=23.4, SD=3.7) which is statistically significant (t(2949)=6.3, p< 0.001). Although there is no difference on average knowledge score between urban and rural and among different age groups, the positive association between education level and average knowledge score is significant (F(5;2495) = 6.1, p< 0.001). Indeed, those with higher education are more likely to have higher average knowledge score. However, between the 1^st and 2^nd round, the increase of average knowledge score among those with lower level of education (high school and below) is significantly greater than those with higher level of education (university and above) (t(2875)=3.9, p< 0.001).

Conclusions:
PHWs consist of pictures and accompanying health warning messages have a certain impact on raising smokers' knowledge on smoking harms. Besides, the PHWs also help reduce the health disparities between groups with different level of education.