There is little research in the developing countries of South East Asia on the effectiveness of bilingual education programmes that use first language instruction for ethnic minority children. This study investigated the effectiveness of a bilingual education programme involving ethnic minority children in Cambodia by comparing their performance in mathematics, Khmer literacy and oral Khmer to their ethnic minority peers whose education is in the national language only. The findings show that students in the bilingual schools performed better in mathematics than their peers in the monolingual schools, but the differences in Khmer literacy and oral Khmer test scores were statistically insignificant. The study suggests that bilingual education using first language instruction could benefit academic development among ethnic minority students in Cambodia, at least in terms of mathematics achievement.