This study is aimed to identify the effects of task complexity on first language (L1) use and the functions it may serve when two groups of learners of English as a foreign language (EFL) work on collaborative tasks. Twenty-four pairs of Chinese EFL learners from two universities were assigned to a lower-proficiency and a higher-proficiency group, each with twelve pairs. Each group completed two collaborative tasks of different cognitive complexity. The results showed that task complexity had an appreciable impact on the use of L1 and its functions. It is also found that the impact of task complexity was dependent on learners' English proficiency. Specifically, the higher-proficiency group employed more L1 to perform complex tasks than they did in the simple versions of the tasks. These learners also devoted significantly more L1 turns to fulfill the functions of metacognitive and grammar talk to complete the complex tasks. These trends did not hold for the lower-proficiency group.