Credit plays a key role for enterprise’s operations and growth in general and for small and medium-sized firms (SMEs) in particular. Numerous policies to connect SMEs to formal credit institutions have been implemented; however the number of SMEs that have not been able to borrow or have not fully obtain a formal loans still accounts a high portion. This research analyzes factors influencing SMEs’ levels of formal credit constraints in Vietnam using the national SME survey in 2015. The multinomial logistic regression model is used to estimate SMEs' credit contrants, the result showed that SMES partially and completely face credit constraints. Of which, year in operations, scale, business registration certificates, relationship with states institutions, participation in economic and social associations, 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.