Building on research that has demonstrated the benefits of Dual-Language Bilingual Education (DLBE) programmes on students' bilingual, academic, and cross-cultural development (Lindholm-Leary and Hernández 2011), this study examines the links between dual language proficiency and competence in elementary students enrolled in a Cantonese DLBE programme in the U.S. Specifically, we examined the relations between (a) children's bilingual (English and Cantonese) language proficiency in four dimensions (reading, writing, listening, and speaking) and (b) their competence in academic, peer relationships, activities involvement, and classroom behaviour domains with a group of 60 fourth and fifth graders enrolled in a Cantonese-English DLBE programme. Multiple regression results show that both Cantonese speaking and writing proficiency had significant main effects on academic competence. These effects remained significant even after controlling for students' English speaking/writing proficiency. Moreover, both Cantonese and English writing proficiency were positively related to students' classroom competence. Additionally, higher English reading proficiency was positively associated with peer competence. Results highlight the different beneficial roles of Cantonese and English proficiency on positive self, peer acceptance, and prosocial behaviour. The current study generates new insights into the role of bilingual proficiency and has implications for DLBE programme curriculum and policy.