This study investigates the effects of the teacher’s use of collocation instruction on Vietnamese English as a Foreign Language (EFL) students’ achievement of collocations in their writing. Twenty-one volunteer students participated in this study. Data were collected through a pre-posttest design and quantitatively analyzed afterwards. The results indicated that the students outperformed in the number and made fewer errors of lexical and grammatical collocations in the post-test compared to the pre-test. Significant correlations between the three collocation types (correct lexical, correct grammatical and incorrect grammatical collocations) and the students’ writing scores were reported, except the type of incorrect lexical collocation. Accordingly, pedagogical implications as well as suggestions for the teachers with regard to teaching collocations in writing classes will be provided.