With the aims to improve government productivity and meet the citizens’ requirements, E-government has gained incredible popularity in many nations worldwide. Vietnam has established an E-government to enhance the quality of its public services for many years. Exploring factors affecting Egovernment adoption in Vietnam has drawn significant attention from many scholars; however, considering the effect of national culture in general, especially the uncertainty avoidance dimension, is still a shortage of studies. Therefore, based on the UTAUT model, this study explores the determinants of usage behavior and the moderating role of uncertainty avoidance dimension to usage behavior of citizens in E-government services in Vietnam. The results show that performance expectancy, effort expectancy, social influence have significant impacts on behavioral intention toward E-government usage behavior, and facilitating conditions play an essential role in forming E-government usage behavior. Moreover, uncertainty avoidance was found to be a moderator to the relationship between social influence and behavioral intention to use E-government.