Vocabulary acquisition is a central component of second language learning. While there have been advancements in our understanding of the factors contributing to vocabulary acquisition in L2 students, it is still unclear how language aptitude is associated with the effects of task involvement load in this process. This study investigates the effects of task involvement load on incidental vocabulary learning and their association with language aptitude. One hundred and forty-four participants were assigned to five groups. All groups finished reading a passage and completed tasks that had different involvement loads. A pretest and two posttests adapted from Paribakht and Wesche's (TESOL Can J 11:9-29, 1993) Vocabulary Knowledge Scale (VKS) were employed to assess learners' development of the target vocabulary items. The LLAMA test (Meara, in: In_lognostics, Retrieved from February 14, 2017, from https://www.lognostics.co.uk/tools/llama, 2005) was administered to measure participants' language aptitude. We found that, in the immediate posttest, the Gap-fill (with "search") group showed superiority over the Reading (no "need") and Reading (with "need") groups, while the Sentence-writing group significantly outperformed the other four groups. In the delayed posttest, both the Gap-fill (with "search") group and the Sentence-writing group significantly outperformed the other three groups. Our study also reveals a moderate correlation between language aptitude and vocabulary learning.