The purpose of this study is to examine whether the difference in involvement load induced by different vocabulary learning tasks affects Korean EFL adult learner` vocabulary short-term learning and long-term retention. This study also investigates how different tasks (Reading, Gap-filling, Unscrambling) with different involvement loads affect vocabulary learning of learners at high proficiency level. For this study, a total of 35 EFL adult learners participated, they were randomly assigned to one of three groups to complete an assigned task. To measure the effectiveness of different task-induced involvement, a pre-test was administered to check both the participants` pre-knowledge regarding the 10 target words and to prove homogeneity of three groups. Then, two post-tests were implemented, one immediately after the treatment and one two weeks after to measure their short-term learning and long-term retention on the target words. For the adult learners, the difference of involvement load was a significant factor in short-term learning, whereas it was partially supported for long-term retention, because Task 2 was more influential than Task 3. Therefore, the present study suggests that a more effective task should be considered for adult learners, who have different amount of time to prepare tests or study English, in teaching environments.