Although cyber stalking has become one of the most serious social issues, there are a few studies on factors related to judgment of cyber stalking. We investigated factors affecting judgment of cyber stalking such as offender-victim relationship and gender. We also explored whether judgment of seriousness of cyber stalking, offender/victim responsibility, and offender/victim blame were affected by interpersonal competence and relational aggression, which is known to be related to interpersonal problem and conflict, and interaction skills. Based on a sample of 160 people (80 males, 80 females) in their twenties to fifties, the study showed that the seriousness of cyber stalking which occurred between strangers than ex-lovers was evaluated more highly, the offender was blamed more, and the responsibility of the victim for the cyber stalking was evaluated lower. The lower the relational aggression, the higher the seriousness of cyber stalking was evaluated, and the more highly blamed the offender. The effect of gender on judgment of cyber stalking was mediated by the level of relational aggression. Women than men were more likely to have a high level of relational aggression, which in turn led to lower judgment of the seriousness of cyber stalking and offender blame. Based on the results, we discussed criminal justice policy implications on cyber stalking victim and limitations of this study.