Abstract: This study employed a 2×2 mixed factorial design to investigate the effects of output task type and word imageability on EFL learners’ vocabulary acquisition. The task type effect was found to be moderated to some extent by word imageability. In receptive vocabulary knowledge, a medium task type effect size was found only at the low level of word imageability, and the word imageability effect was found across the tasks, but with different effect sizes. In productive vocabulary knowledge, however, a large task type effect size was found at each level of word imageability, and a low medium word imageability effect size was found only for sentence combining. It is concluded that, apart from mode of word processing, division of attention, word imageability and type of vocabulary knowledge all affect vocabulary acquisition.