Abstract: The present study focuses on the identifying subject's mentality facticity during concealed information test (CIT). Total 15 subjects were tested in CIT paradigm and the Event Related Potential (ERP) evoked by three categories stimuli was obtained simultaneously. The results show that the Approximate Entropy (ApEn) of ERPs decrease as stimulus appears and increase as stimulus over. Meanwhile, decreasing percentage of ApEn was employed as quantitive parameter, it is found that the parameter of ERPs corresponding to irrelevant stimulus is significantly larger than that of probe and target stimulus (p＜0.001). The findings indicate that decreasing percentage of ApEn could be used as an additional feature parameter to identify subject's concealed information effectively.