In response to little known about the use of face masks for public transport passengers in developing countries, this study investigates the prevalence of using masks and using masks correctly together with influential factors. Using 570-passenger data collected on the whole bus network from 7 September to 3 October 2020 in Hanoi, the authors found that 100% of users wore masks; however, about 11% failed to wear masks correctly. As regards factors, passengers who are old, rarely ride by bus, take heavy luggage, travel with other(s) were more likely to use masks incorrectly. Having a health issue encouraged the correct use of masks. Attitudes towards the COVID-19 were significant factors. The higher levels of agreement with the acute danger of COVID-19 and the risk of infection from the public were involved in the higher likelihood of the correct mask wearing. Notably, over time with no community transmission of the coronavirus, the likelihood of incorrect use of masks was more inclined to increase. To address the wrong utilization of masks on buses, the role of ticket conductors in reminding and asking users using masks incorrectly should be enhanced. Additionally, authorities should issue messages and implement campaigns to encourage citizens to wear in public spaces on a regular basis, particularly when the adherence reduces over the time without community transmission of COVID-19. Although not covering all aspects representing the incorrect mask wearing, this study is one the first research on the incorrect use of masks, thus extending the literature on how public transport users respond the effects of COVID-19.