The purpose of this study was to examine the moderating effect of mindfulness in romantic relationships, between irrational relationship beliefs, aggression. The sample randomized online and offline questionnaire survey of 304 unmarried men and women over 20 years old. From this survey, 297 participants except 7 who had no experience of romantic relationships were analyzed. Used measures were as follows: the irrational relationship belief scale, aggression scale and mindfulness scale. The results of this study are as follows. First, the higher the irrational relationship belief, the higher the aggression significant the main effect was. Second, the moderated effect of mindfulness was significant in the effect of irrational relatioship beliefs on aggression. Also aggression is different depending on the level of mindfulness. Through this study, we indicate mindfulness effectively moderated the aggression of the who has an irrational realtionship beliefs.