Abstract: This paper presents a methodology for modeling and quantitative reliability assessment of nuclear safety-level digital I＆C system software based on Multilevel Flow Models（MFM）.By developing a MFM graphical modeling and analysis platform,this paper presents a MFM model for a PID control software of DCS and performs software reliability assessment based on test results.Using the proposed method of this paper,the reliability of main goal and all sub-goals of software can be obtained by only one calculation and the weak points in the software design are very convenient to be identified.In addition,the software models are easy to build and modify.