The construction industry should seek to enrich its workforces due to the increasing lack of trained and skilled employees. This study attempts to investigate the relationship between encouragement-based management mechanism (EMM) and manpower development (MD) in construction firms and examine the moderating effect of firm size on this relationship by conducting bivariate Pearson correlation and hierarchical multiple regression analyses to analyse empirical data collected from 79 construction firms in Vietnam. The results show that EMM positively affected MD, and this positive effect was stronger in small/medium-sized construction firms than in large firms. The findings could provide construction firms in Vietnam, as well as other developing countries, with a better understanding of the effect of EMM on MD. Hence, they could establish appropriate and wise encouragement-based strategies to enhance their manpower. This study could contribute to the extant literature on construction manpower development by providing empirical evidence of the EMM−MD relationship in the context of construction firms.