Capital occupies a very important role for the existence and development of enterprises in general and small and medium-sized firms in particular. In our country, many policies have been put in to connect business and capital of formal credit institutions. However, the number of firms that has not approached these loans and has looked for informal lenders still accounts for a high proportion. This research analyses factors influencing the formal credit constraints of small and medium enterprises in Vietnam. The findings show that age of enterprise, size of enterprise, business registration certificates, relationship with governmental institutions, participation in economic and social association, undue repayment, lenders of the enterprise, short-term maturity, not to apply for any loans yet, and loan guarantor can decrease the possibility of the formal credit constraints of SMEs. In contrast, age of owner/manager, refusal of formal loans, having informal loans, and not to have collateral assets increase the likelihood of formal credit constraints. In particular, the study also shows the differences of effect of factors between the cases of partial-credit-constraint and complete-credit-constraint.