This paper investigates how electronic word of mouth (eWOM) campaigns with rewards influence customer intention to post a review as eWOM givers and eWOM credibility with receivers. Recently, eWOM campaigns in which manufacturers and retailers give some rewards to customers for posting a review have come to be often seen on their websites. Although a considerable amount of research has been conducted focusing on eWOM, few studies treat this kind of campaigns. In this study, we utilized the cost-benefit theory to examine what kind of reward of eWOM campaigns may increase consumer intention to post a review. We identified three reward characteristics―monetary value, availability in market, and winning rate—and proposed three hypotheses. With a laboratory experiment using a fictitious e-retailer’s website with an eWOM campaign, we asked participants about their intentions to post reviews. The results of ANOVA showed that consumer intention to post a review is higher when the reward has higher value, is difficult to get in the market, and/or has higher winning rate. On the other hand, we utilized attribution theory to investigate eWOM credibility with eWOM campaigns. We presented fictitious e-retailer’s websites with/without an eWOM campaign to respondents and asked them the degree of credibility of eWOM. The result of t-test suggested that when reviews are perceived to be posted as a result of eWOM campaigns, receivers could perceive eWOM credibility to be lower. This study indicates that the reward with higher value increases the amount of reviews. However, a website with an eWOM campaign makes the eWOM less trustworthy. Manufacturers and retailers should try to subtly display the information of eWOM campaign. Otherwise, an eWOM campaign may become an ineffective way of promotion.