Software quality is the integration of a set of software inherent characteristics to meet users' needs. An inverse process approach of quality function deployment to assess software quality is proposed based on users' needs. Customer satisfaction is measured according to the given quality characteristic. The process and mathematic model to measure software quality is shown. The empirical study indicates that the method is effective in assessing software quality. According to users' needs and the evaluating results, software quality and customers' satisfaction degree are continuously improved.