Objectives : N100 amplitude slope(the intensity dependence of the cortical auditory evoked potentials) is widely considered as an indirect indicator of central serotonergic neurotransmission. However, there are only a few studies about N100 amplitude slopes of major psychiatric disorders. In this study, we examined N100 amplitude slope differences among major depressive disorder(MDD), bipolar disorder(BD), schizophrenia (SCZ) and normal controls(NC). Methods : We measured the N100 amplitude slopes of 35 patients with MDD, 33 patients with BD, 27 patients with SCZ and 35 NC subjects. Amplitude differences from N1 to P2 at the five different sound intensities(55, 65, 75, 85 and 95dB) were examined at Cz electrode. The N100 amplitude slope was calculated as the linear regression of five N1/P2 peak-to-peak amplitudes across stimulus intensities. Results : BD patients showed significantly reduced N100 amplitude slope compared with NC(0.54${pm}$0.70 vs. 0.96${pm}$0.72, p=0.035). N100 amplitude slope of SCZ patients was significantly reduced compared with NC(0.50${pm}$0.47 vs. 0.96${pm}$0.72, p=0.027). N100 amplitude slope of BD patients was significantly lower than that of MDD patients(0.54${pm}$0.70 vs. 0.94${pm}$0.60, p=0.046). SCZ patients also showed significant reduction of N100 amplitude slope compared with MDD patients(0.50${pm}$0.47 vs. 0.94${pm}$0.60, p=0.036). There was no significant difference of N100 amplitude slope between MDD patients and NC(0.94${pm}$0.60 vs. 0.96${pm}$0.72, p=1.000). Conclusion : Interestingly, the N100 amplitude slopes of BD and SCZ were reduced compared to NC and MDD patients. Our results suggest the predictive use of N100 amplitude slope in making differential diagnoses of major psychiatric disorders. Clinical implications of N100 amplitude slope in major psychiatric disorders were discussed.