Objectives: To investigate a method for predicting postoperative facial nerve palsy during parotid surgery using facial nerve monitoring. Design and setting: This was a prospective study of diagnostic tests performed from 2015 to 2018. Participants: We included adult patients who were underwent parotid surgery. Main outcome measure: We assessed prediction for postoperative facial nerve palsy by using intraoperative facial nerve monitoring, comparing between facial nerve trunk stimulation before and after tumor resection and between stimulation in the facial nerve trunk and each branch by using facial nerve monitoring. The amplitude response ratio (ARR) was calculated for the nerve trunk before/after surgery (ARR1) and for the trunk/periphery (ARR2). We compared these data in each group. In addition, we then examined the correlation between ARR and time to recovery of paralyzed branches. Results: A total of 113 patients were included. Among the 113 patients, 372 branches of 93 patients did not develop facial nerve palsy and were classified as group A. Among 20 patients who developed paralysis, 51 branches without paralysis were classified as group B, and 29 branches with paralysis were classified as group C. In patients with postoperative facial nerve palsy, the potential of the main stimulus was lower after removal than before removal. Similarly, the main trunk stimulation was lower than the peripheral stimulation after tumor extraction. When cut-off values for ARR1 and ARR2 were set to 0.63 and 0.55, respectively, the accuracy of postoperative facial nerve palsy diagnosis was 91.7% and 96.0%, respectively. Conclusion: Using FNM during parotid surgery enables easy prediction of postoperative facial nerve palsy.