Vocabulary research has been done in numerous ways in the area of Korean language education. However, the related variables of vocabulary acquisition have never been searched for based on the second language theory. In particular, there has been a lack of focus on the gap between receptive and productive vocabulary. This study investigates the vocabulary size of Korean language learners to measure the difference between receptive and productive vocabulary. A total of 66 Korean language learners of academic purposes participated in the receptive and productive test of Korean vocabulary made by the researcher. Statistical analysis was done, which showed the mean for receptive vocabulary was 12.59 (out of 15) compared to just 9.41 for productive vocabulary. Two main variables-how long a participant has been learning Korean, and the achievement score from the participants’ Korean writing class--were entered into the correlation analysis. Receptive vocabulary correlated at r=.45, r=.36 respectively for the duration and achivement. However, productive vocabulary only correlated at r=.55 for the achievement score. The result of this study implies that the process of two kinds of vocabulary acquisition follows different routes. There appear to be other related variables in acquiring productive vocabulary which could have been researched. Therefore, a future study needs to find other related variables to enlighten the process of vocabulary acquisition. (Yonsei University)