Redundancy techniques are widely used to design fault tolerant systems. Diversity has long been used to protect redundant systems from Common Cause Failure (CCF). Whilst there is clear evidence that diversity can bring benefits, these benefits can be difficult to quantify. Therefore, a novel method which researches the effect of diversity on CCF mitigation from the component degradation state point of view is proposed in this paper. Four categories of impact factors (IFs), which influence the failure behavior of the component, are defined and the corresponding method of mapping diversity into component degradation valve (CDV) has been proposed aiming at the characteristics of each IF category. Then CCF analysis is performed based on weighted impact vectors which can be calculated from CDVs. Due to related reliability data has been collected for each IF category the method has a good applying prospect. An illustrative case study is presented, which shows the application process of the novel method.