Objective · To explore the relationship between θ oscillation of auditory event-related potential P300 and clinical symptoms in the patients with clinical high risk (CHR) of psychosis or first episode schizophrenia (FES). Methods · A total of 18 patients with CHR of psychosis (CHR group), 18 patients with FES (FES group) and 19 healthy controls (control group) were recruited to complete the standard auditory Oddball paradigm as well as electroencephalogram (EEG) recording with 64-channel EEG cap. Fz, Cz and Pz electrodes were selected to compare the amplitudes and peak latent periods among the three groups, and time-frequency analysis of the θ oscillation in P300 was performed. Results · There was significant difference in the amplitudes of P300 among the three groups (P0.004). The amplitudes of P300 in FES group (P0.001) and CHR group (P0.026) were both significantly lower than that of control group. There was no difference between CHR group and FES group in P300 amplitude (P>0.05). θ Oscillation induced power was significantly different among the three groups (P0.022). The induced power value of FES group was significantly lower than that of control group (P0.008); and it was marginally lower in CHR group as compared to that of control group (P0.054). There was no difference between CHR group and FES group (P>0.05). There was trending difference in θ oscillation evoked power among the three groups (P0.054). The correlation analysis showed that the total and subscale scores of Positive and Negative Syndrome Scale in FES group were significantly correlated with the amplitudes of P300 at Cz and Pz electrodes (P<0.05). Conclusion · There are significant abnormalities in the amplitude of auditory P300 and θ oscillation of P300 in both CHR and FES patients; however, the change in CHR patients is less severe than that of FES patients. The impaired θ oscillation of P300 in CHR patients is limited to induced power, but evoked power is not impaired.