Most of the existing studies on open source software (OSS) have been focusing on software development process while how and why OSS is accepted by software developers is yet to be uncovered. This paper adopts expectancy disconfirmation model to explain the process and major factors of user satisfaction on OSS. An empirical research based on questionnaire survey is performed to study the influence of user expectancy to user satisfaction of commercial and open source DBMSs. According to the survey results, reputation and past experiences influence significantly to user expectancy on DBMS, and price influences to user expectancy only for commercial DBMS users. The results also show that the expectancy disconfirmation influences significantly to user satisfaction of both types of DBMS. However, the degree of expectancy disconfirmation does not have linear relationship with the degree of user satisfaction.