This study investigated the influence of focus-on-form task-based language teaching (TBLT) on Taiwanese children's English vocabulary acquisition and retention. Focus-on-form TBLT here refers to instruction in which the teacher and students interact, negotiate, and respond to each other on the subject of a second language (L2) vocabulary. The participants (N = 71) were all enrolled in the third grade of a central Taiwan elementary school. The experimental group (N = 42) received TBLT lessons with a focus on form. In contrast, the control group (N = 29) received more conventional lessons based on the presentation-practice-production (PPP) model. Quantitative data were collected from three vocabulary tests. Qualitative data were gleaned from the teacher/researcher's journal logs. Although the statistical comparisons showed no significant differences between the two groups in the three VKS tests, the qualitative data suggest that students in the two groups responded differently in terms of their in-class interaction and personal involvement. It seems that interaction and output production induced in the experimental group possibly facilitated the comprehension and acquisition of L2 vocabulary. The study also provides pedagogical implications for implementing TBLT with a focus on form to increase the retention of L2 vocabulary.