The present study was conducted to investigate the intergroup behaviour from the perspective of social identity theory. It was predicted that (a) when group membership was based on trivial categorization (e.g., by drawing lots), minority group members would be more conscious of their social identity, and favour their own group more in reward distribution than majority group members; (b) when based on value-loaded categorization (e.g., by social attitudes), both minority group and majority group members would favour their own group; (c) both minority group and majority group members would perceive converted members, who move away from their initial attitudes, as a threat to their social identity, and discriminate them. Results from three experiments under the minimal group paradigm, with undergraduate students, supported these predictions. Findings were discussed in terms of salience of social identity in categorization of minority versus majaority, and the impact of anonymity in the minimal group paradigm. They were also discussed to compare the theory with belief congruence theory, which argues that attraction due to similarity of belief is the cause of ingroup favouritism.