Objective To observe the latency and amplitude characteristics of event-related potential P300 under different target stimulus. Methods During January to December, 2014, 35 patients with memory impairment received P300 test under the percentage of target stimuli of 0.10 and 0.25. The latency, amplitude and area under amplitude were compared. Results There was no significant difference in latency (t<0.256, P>0.800), but the amplitude and area under amplitude increased under 0.10 compared with those under 0.25 (t>4.259, P< 0.05). Conclusion Less target stimulation may improve the patients' attention and increase amplitude of P300, but no more interference in latency, suggesting latency of P300 is more stable for assessment of cognitive function.