【Abstract】 Objective To study cognition function in uremic patients and the changes of cognition function after hemodialysis and hemodiafiltration by examination of event-related potentials P300. Methods P300 was determined in 31 patients treated with hemodialysis, 21 patients treated with heomdiafiltration and 49 normal controls. Results P300 was abnormal in 86.6% uremic patients. The P300 demonstrated prolonged latency and decreased amplitude as compared with those of the control group. Conclusions Cognition function was abnormal in most uremic patients Blood purification, especially heomdiafiltration, improves cognition function in these patients.