Objective To investigate the event-related potentials (ERP), P300, mismatch negativity (MMN), and contingent negative variation (CNV) in the elderly patients with cognitive impairment, including Alzheimer’s disease (AD) and mild cognitive impairment (MCI), to evaluate the diagnostic value of the 3 ERP in patients with MCI. Methods A total of 80 cognitive impairment in- and out-patients with age over 60 years in our department from February 2011 to March 2013 were enrolled in this study, and they were divided into mild AD group (n＝40) and MCI group (n＝40). Another 40 individuals with normal cognition served as normal control. Peak latencies (PL) and amplitudes (Amp) of the ERP, including P300, MMN, CNVM1, and CNVM2 were measured. Results Compared with the NC group, the PL of ERP were significantly delayed in the AD group (P＜0.05). The PL of P300 [(380.94±37.55)ms] and MMN [(188.63±31.63)ms] in the MCI group were longer than those of the NC group [(342.88±41.72) vs (137.48±28.69)ms; P＜0.05]. However, significant difference was not found in PL of CNVM1 and CNVM2 between MCI group and NC group (P＞0.05). Compared with the NC group, the Amp of the three ERP in the AD group were significantly shorter (P＜0.05). The Amp of MMN and CNVM2 in the MCI group were shorter than those in the NC group (P＜0.05). There was no significant difference in the Amp of the 3 ERP between the AD and MCI groups (P＞0.05). Conclusion P300, MMN and CNV (CNVM1 and CNVM2) are of value in the diagnosis of MCI. P300 and MMN are superior to CNV in distinguishing MCI from normal cognitive function.