Evaluation is an important and basic issue for software dependability. However, traditional researches mostly focused on the conceptions and properties of dependability, which resulted in lots of different views about software dependability. Based upon Pi Calculus, dependable action path of software and real action path are set up through series of actions, which is decomposed by running software behaviors. In addition, three kinds of relationships between behaviors are defined. Consequently, the dependability is calculated by comparing the real running software actions to the decomposed ones based on the defined relationships. Experimental result conducted by MWB shows that the proposed method in one behavior is reasonable. At last, a real experiment proves the proposed measure method is more valid than traditional one.