This research study investigated the influence of learners' mother tongue on their acquisition of English collocations. Having drawn the linguistic data from two groups of Thai EFL learners differing in English proficiency level, the researcher found that the native language (L1) plays a significant role in the participants' collocation learning as it is regarded as a primary learning strategy on which they depend. Such L1 transfer seems to lead the learners to a number of collocational problems in the target language (TL), e.g. preposition omission, preposition insertion, non-targetlike word choice, and collocate redundancy. It was discovered that high-proficiency learners relied heavily upon their L1, a behavior claimed to be characteristic of those with limited TL knowledge (Ellis, 2008; Odlin, 1989). That the L1 evidently has a great impact on high-proficiency learners' use of English collocations found support for some past studies (e.g. Boonyasaquan, 2006; Koya, 2003).