The purpose of the current study was to investigate the relationship between frequency values of multi-word expressions (MWEs) and the writing proficiency and overall proficiency of tertiary level English as a Foreign Language (EFL) students. The correlational study was conducted with 63 learners at the school of foreign languages of a state university in Ankara, Turkey. Having collected the essays written by the learners, the researcher applied a two-step item extraction method. After MWEs in the essays were tagged by two experienced raters, frequency-based association strength values were assigned to each phrase. The association strength values were obtained through a reference corpus to determine the MWEs accepted for regression analyses. The results indicated that proportion and mutual information scores of MWEs were significant predictors of the writing proficiency of the learners. Another finding of the study was that no relationship was found between proportion and mutual information scores of MWEs and the overall proficiency of the learners. As MWE frequency measures were predictive of writing proficiency, inclusion of MWEs thoroughly in EFL curricula might be suggested as an implication.