To study the mechanism underlying the influence of psychological pressure on task performance, we investigated the relationship between prefrontal activation, autonomic arousal, and performance in an n-back working memory task with 3 load levels (l-, 2-, and 3-back tasks) under evaluative pressure. The tasks were performed by 32 university students with or without evaluative observation by experimenters. The error rate and prefrontal activation were found to increase with pressure only in the highest load task (3-back). In contrast, autonomic arousal increased with pressure regardless of the task condition. Correlation analysis showed a positive correlation of the error rate with prefrontal activation in the 3-back task and no consistent correlation with autonomic arousal. We concluded that the inhibitory effect of evaluative pressure on task performance is mediated by prefrontal overactivation rather than autonomic arousal.