Research has demonstrated that second language immersion is an effective means of facilitating primary school students' second language without undermining competence in their first language. Despite the rapid growth of English immersion (EI) programmes in China, only limited empirical research has been conducted to evaluate students' academic achievement in these programmes. This study addressed three primary research questions regarding EI students' academic achievement represented by English (L2), Chinese (L1) and mathematics. This study was conducted with a group of Grade 2 (n = 385), Grade 4 (n = 430) and Grade 6 (n = 183) students in immersion or non-immersion programmes in three schools in China. Cambridge Young Learners English Tests were employed as the L2 measure. School-issued achievement tests in L1 (Chinese) and in mathematics were also employed. The results show that immersion students, compared with non-immersion students, did better in English at all three grade levels. They also did similarly in Chinese and mathematics at Grades 2 and 4, but better at Grade 6. The findings from this evaluation study demonstrate a complex and developmental picture of students' academic achievement in English, Chinese and mathematics. (Contains 4 tables, 1 figure, and 1 note.)