The development of L2 utterance fluency has been extensively researched, whereas that of cognitive fluency has rarely been examined. This study investigated the longitudinal development of L2 utterance and cognitive fluency and their relationship. Thirty-one Chinese learners of English completed speaking tasks and a set of tasks for cognitive fluency before and after 5 months' study abroad. The results showed that participants made a significant improvement in mean syllable duration, end-clause pause frequency, and the speed of syntactic encoding and articulation but not in mid-clause pause frequency or lexical retrieval speed. Mixed-effects modeling confirmed a significant relationship between syntactic encoding speed and mean syllable duration and mid-clause pausing. Furthermore, the significant relationships were maintained over time. The findings highlight (a) the differences between mid-clause and end-clause pausing in terms of their developmental patterns and relationship with cognitive fluency and (b) a significant role of syntactic encoding speed in L2 utterance fluency.