Abstract: In this paper,the main challenges of software development and current situation of software process application are introduced, followed by the study of two software process improvement approach, top down and bottom up. The top down approach is based on SW CMM process framework and directed by its five level structure. The bottom up is product based process improvement. There are three sub processes in bottom up software process improvement model: “understanding”, “assessing”, and “packaging”. The baseline of software process is established in “understanding” stage. Process changes are imported and their impact is evaluated in “assessing” stage. The successful practices are packaged in the “packaging” stage. Then, the comparisons between two approaches are discussed on four aspects:software process improvement goals, software process baseline, software process change assessing, and software process improvement approach.