Objective To investigate the relationship between cognitive function and event-related potential P300 in male long-term hospitalized patients with schizophrenia. Method Totals of 82 male long-term hospitalized patients with schizophrenia and 52 healthy controls were recruited. The event-related potential P300 was analyzed by the United States Nicolet Viking Quest evoked potential instrument.The cognitive function was tested by animal naming test，category fluency test，digital cancellation test，trail taking test (TMT-A，TMT-B)，stroop test (word，colour，word-colour interference test)，block design test and WMS-Ⅲspatial span test. Results There were significant differences between patient group and controlgroup in all items of cognitive function assessment scales (P＜0.01) . There were longer latency of P300 (t=22.990，P ＜0.01) and lower amplitude of P300 (t=-9.699，P ＜0.01) in patient group than that in control group. The P300latency was positively correlated with digital cancellation test and the score of TMT-A in patient group (r=0.481，P＜0.01；r=0.245，P＜0.05). Moreover，P300 amplitude was negatively correlated with digital cancellation test in patient group (r= -0.338，P＜0.01). Conclusion The male long-term hospitalized patients with schizophrenia still have cognitive impairment，even when their psychiatric symptoms are stable. The latency and amplitude of event-relatedpotential P300 may be an electrophysiological marker of cognitive function in schizophrenic patients，which were correlated with the results in the cognitive assessment scales.