This study examined the effects of task types on English as a foreign language (EFL) learners' receptive and productive vocabulary knowledge. To this end, 130 (70 female and 60 male) EFL learners were randomly assigned to one of six tasks of learning 30 target words. The design of the tasks was based on the involvement load hypothesis (ILH) arguing that learning of unfamiliar words to be contingent on the amount of task induced involvement. The components of involvement in ILH include need (N), search (S), and evaluation (E). In this study, the tasks induced the same or different involvement loads regarding the presence and strength of each component: paragraph writing (+N, +S, ++E), sentence writing (+N, +S, +E), combining (+N, -S, +E), fill in the blank (+N, -S, +E), translation (+N, -S, +E), and control (-N, -S, -E). After the last treatment session, both receptive and productive knowledge of the target words were measured. Moreover, a delayed posttest was administered 1 month later. The results revealed that all output tasks were more effective than the control task in enhancing the participants' receptive and productive vocabulary knowledge. Moreover, paragraph writing task was found to be the most effective task.