The main aim of teaching the Malay language at school is to enable students to master the grammar of the language. Mastering the language means students’ competency in understanding, knowing, identifying and using the rules of the Malay grammar correctly. Malay idiom is a tool that depicts the rich poetic imagination with rhetoric aspect and not just beautiful and creative language. Thus, this article describes and explains the achievement of non-Malay secondary students in Kedah, Negeri Sembilan, and Sarawak, in mastering the Malay language grammar system that covers the use of idioms. The subjects consist of 300 Form 4 students in the three states. The research findings show that in general, the achievement of non-Malays is weak in terms of the correct use of idioms. In fact, the use of idioms among young non-Malays is still weak, particularly in understanding intended meanings and thus needs a serious attention. The finding also shows that the achievement of language among non-Malay students has a significant relation with their achievement in mastering grammar.