The purpose of this study is to investigate the effects of using and authoring web-based vocabulary learning tasks on Korean EFL learners' vocabulary acquisition. The subjects of the study were seventy-seven EFL college students in Korea. They were divided into three groups: a traditional control group, and two experimental groups using and authoring the web-based vocabulary tasks, respectively. The major results of the study were as follows: first, the two experimental groups performed significantly better on the vocabulary recognition and use posttest than the control group did. This result indicates that both the use of the web-based tasks and the participation of students in authoring the web-based tasks are more effective in vocabulary acquisition than the use of traditional vocabulary learning methods. Second, both the experimental groups performed significantly better on the delayed posttest than the control group did. This result provides evidence that using and authoring web-based vocabulary tasks are effective in helping students retain newly acquired words in the long-term memory. Third, the learners' responses of the two experimental groups prove to be positive with respect to the use of the web-based task authoring tools in vocabulary learning. Based on these results, some pedagogical suggestions should be made on the effective use of the web-based task authoring tools in Korean English classroom settings.