This study examines the effects of reading and writing activities on English vocabulary learning. Sixty high school students participated in the study and were randomly assigned to one of either reading activity or writing activity groups with 30 students in each. The learners’ knowledge of English vocabulary was measured through pre-, post-, and delayed tests under multiple treatment sessions. The findings of this study demonstrate that the two types of classroom activities positively influenced Korean high school students’ lexical knowledge, showing the necessity of vocabulary teaching along with other language skills. The results also reveal that the learners in the writing activity group exhibited significantly higher vocabulary knowledge gains in long- as well as short-term retention. Pedagogical implications were made based on the results