This article reviews past studies on effects of humor on persuasion and the mechanism involved. First, three methodological problems in past studies were indicated (defects in design, defects in procedure, defects in analysis). When these studies were excluded, the remaining studies showed the followings. Most of the studies reported no main effect of humor on persuasion, but there are a few studies that showed positive main effects (facilitation effect). However, there is no study that showed negative main effects (inhibition effect). Moreover, it became evident that humor factor interacted with eight factors. The interaction effects were either facilitation or inhibition of persuasive effects. This result was explained by Elaboration Likelihood Model (Petty & Cacioppo, 1986b). As for the mechanism, the facilitation effects on attention to message, evaluation of message, and liking of sender as mediating factors, and the facilitation effects on positive mood of recipients were supported. In addition, the mechanism was discussed from the viewpoint of information processing.Finally, it was suggested that future studies should investigate the following three points: 1) effects of the types and the amount of humor stimulus on persuasion, 2) the difference of persuasive effects by sense of humor of recipients, and 3) further investigation of the mechanism from the information processing point of view.