To reduce the care burden of dementia, identifying whether the combined effect of metabolic syndrome and epsilon4 increases the risk of cognitive decline needs to be determined. Using the Cognitive Abilities Screening Instrument (CASI), 145 mentally healthy middle-aged and older adults were recruited to investigate the influence of metabolic syndrome and epsilon4 on cognitive event-related potentials (ERPs). The results showed no difference in CASI scores, N100 and P300 measurements and epsilon4 carrier percentage between participants with and without metabolic syndrome. The epsilon4 carriers displayed a significant decrease in P300 amplitude, although the CASI scores and N100 component showed no difference. We conclude that metabolic syndrome exerts little effect on N100 and P300 measurements, and that epsilon4 carrier is an independent predictor of low P300 amplitude.