This study aimed to investigate the effect of explicit teaching of language chunks on the writing performance of Iranian English as a foreign language (EFL) learners. A total of 42 low-intermediate homogeneous female learners aged between 13 and 14 years from a junior high school, provided the study with randomly assigned control group (N=22) and an experimental group (N=20). During 20 sessions of instruction, language chunks in short paragraphs, dialogues, and reading texts were focused on only on the experimental group. The language chunks and their first language (L1) equivalents were specified in order to determine the boundaries of chunks to convey an intended message. Development of learners' writings was examined through writing tasks that were administered twice - before and after the treatment. Statistical analyses of independent samples t-test revealed significant improvement of the experimental group in their production and overall general writing, whereas moderate improvement was observed in the control group. Hence, the role of chunking strategy in language instruction domain has the potential to improve the writing quality of learners. The findings of this study can be beneficial for learners, teachers, teacher educators, and syllabus designers in foreign language (FL) contexts so as to improve the learners' language production.