Discretionary behaviors of students that contribute to their university are termed university citizenship behavior (UCB). Past research has shown that students regarded their diligence in class to be a part of UCB, similar to conscientiousness of employees being a part of organizational citizenship behavior (OCB). This study examines the effect of professors' lecture justice behavior (fair-minded teaching behaviors) on students' diligence in class. Data were collected from 385 students at a private university in Tokyo, Japan. The results indicated that lecture justice had a positive effect on UCB in class, and this effect was fully mediated by satisfaction. Some implications for practice and future research are also discussed.