One widely accepted explanation of bullying, known as the aggressive-motive thesis, assumes that bullying is a form of aggressive behaviour triggered by external stress. However, recent evidences have suggested a different explanation, known as the frustration-aggression thesis, which asserts that bullying is a psychological defense triggered by external stress to reduce anxiety. The present investigation is an attempt to compare school bullying in Japan and Hong Kong and to find out whether the frustration-aggression thesis is applicable in the schooling context of both societies. Data analysis using structural equation modeling was based on 703 girls selected from 13 primary and 6 junior secondary schools in Japan and 2,477 girls selected from 21 primary and 19 secondary schools in Hong Kong. Results of the analysis suggest that girl bullying in Japan can be explained by the frustration-aggression thesis but that girl bullying in Hong Kong can be explained by the aggressive-motive thesis. The reasons behind the different bullying mechanisms are suggested, but further investigations are needed to uncover the underlying causes. (Contains 4 figures and 3 tables.)