The use of collocation has always been considered as a good yardstick by which the idiomaticity of second language (L2) use is reliably judged. Hence, this study aims at investigating the relationship between optimizing the lexical approach and improving the collocational accuracy and the ability to operate on the idiom principle in the writing of Algerian English Freshmen. To carry out this research, an experimental and a control group were assigned for a quasi-classroom experiment. While the experimental group was taught collocations based on the principled practices of the lexical approach, the control group was taught conventionally with on special attention paid to collocations. Data was collected from 124 compositions (pre/posttests) done by these learners. Results of this investigation show that there is a correlation between training students to chunk language successfully and the increase of collocational strength with high mutual information (MI) scores in their writing. Additionally, the chunk-based instructional program helped the experimental group develop a habit of processing language as building blocks and this, in turn, reflects these students’ tendency to operate on the idiom principle. Correspondingly, the paper concludes with pointing out some implications for effective acquisition of L2 lexis and future horizons related to developing the idiomacity of L2 writing.