This study explored the role of lexical bundles in the language development of L2 learners; the frequency, structural forms (patterns), and functions of formulaic utterances used by Korean English learners in a controlled environment across different proficiency levels were examined. Conscientious procedures were taken to identify and analyze lexical bundles of L2 learners' spoken data, since few studies have examined these, while some have compared written data. The oral response of 58 Korean English learners were transcribed and compared across three levels (novice, intermediate, and advanced). This study seemed to confirm the findings of previous studies, in that there could be an upward trend in language development as proficiency increases. It is worth noting the participants used lexical bundles in a limited manner similarly across the three proficiency levels, which was less frequent and less diverse in terms of structural and functional patterns when they had to produce oral data rather than written data. The results can be of significant help for learners to improve their English speaking, and for teachers to diagnose their students' speaking proficiency effectively, and should thus be integrated in English teaching curricula. The details of the results and implications are provided.