A growing body of research has indicated that a lexical bundle approach is effective in capturing the phraseological variation in texts across registers and writing proficiency levels. However, research to date has rarely employed such an approach to analyze small-scale learner writing produced in classroom-based L2 research. In response to Reppen's (2016) call for promoting the connection between corpus linguistics and language teaching, this study employed a lexical bundle approach to examine the phraseological variation in learners' co-constructed texts produced in collaborative writing tasks implemented in two task conditions (L1 versus L2 interaction) in EFL classrooms. The results indicated that a lexical bundle approach was able to identify similarities and nuanced differences in the use of formulaic linguistic patterns in a small collection of learner writing produced in the two task conditions. Specifically, learners in the L2 interaction task condition employed more verb phrase based formulaic expressions in their collaborative texts than those in the L1 interaction condition, which may have rendered their texts less academic-like. In addition, learners relied more on longer strings of words (5-word lexical bundles) to express obligation and call for actions in the L2 interaction condition. Methodological and pedagogical implications were discussed.