Objective: To study the characteristics of P300 and topographic dist ribution mapping in obsessive-compulsive patients. Methods: The P300 and topog raphic distribution mapping were recorded in 36 patients, using a Bneuro Galileoinstrument. Results: Compared with normal subjects, the wa ve patterns of obsessi ve-compulsive patients were unstable; the frontal wave variation and dissymmetry between the two sides was 63.9%; the N2 and P3 latency was prolonged; the P2 an d P3 amplitude was decreased; the P3 topographic distribution mapping was uneven ly distributed, the normal high amplitude in the parietal region was absent. For the patient group, energy levels below grade 5 in the left brain area, the fron tal area, and for both were 42.3%, 30.3%, 15.15% respectively. Conclu sion: P300 and topographic distribution might be served as an objective index for reflectin g cognitive activity in obsessive-compulsive patients.