Previous research reports the positive impact of dual immersion programs on students' language proficiency, yet fewer studies have examined students' attitudes in such programs. The current study examined student attitudes and language proficiency in a Cantonese dual immersion school and explored relationships between students' attitudes and language proficiency in Cantonese. Language proficiency results showed that, on average, Cantonese immersion student performances indicated attainment of Intermediate-High level in listening, Novice-Mid level in reading, and Novice-High level in writing and speaking. In addition, heritage learners outperformed nonheritage learners in reading and listening skills. Survey data revealed that heritage learners held more positive attitudes toward learning Cantonese compared with nonheritage learners, but there were no differences in attitudes toward the focal school between the two student groups. Regression analysis revealed that students' attitudes toward the dual immersion school explained significant variance in their Cantonese reading, listening, and speaking skills. Qualitative data also revealed that students' positive attitudes toward their school were shaped by their experiences with their principal, teachers, and school activities. These findings shed light on the role that positive school and learning environments play in helping dual language immersion school students acquire proficiency in the partner language.