The purpose of this study is to investigate the positive and negative effects of teachers' humor and research about conditions and strategies for effective use. First, presents the concept of teacher's humor. Positive effects of teacher's humor are as following: students's resistance decrease, intimate relation formation with a student, attention concentration and raised awareness, increase of memory and recall, improvement in creativity and critical thinking. Even though humor has a lot of merits, humor has some limitations. If a teacher uses humor insincerely, for instance atrocious punning and suggestive humor, negative humor, aggressive humor ect., using humor is to be able to feel a awkward atmosphere, a sense of distance with teacher. Therefore, effective use of humor must tied in effective and well-directed conditions and strategies. Effective conditions for humor using is to fitness on student's cognitive development, student's sympathy based on their experience, a close relaxation with students, teacher's well humor sense. And effective methods for humor using is that teacher know humor's positive and negative effects, make a thorough using plan, going on to ad-lib a heartwarming humor.