This paper examined factors affecting Korean EFL learners' word association types in their L2 mental lexicon. Specifically, vocabulary size of the learners was examined to see if it had any significant relationship with word association types. To this end, experiment procedures that included vocabulary size test and the lexical decision task as well as the word association task were conducted on 40 Korean EFL learners. Reaction time and accuracy of responding to word associations in the lexical decision task were measured. Subsequently, a correlation analysis was conducted with their vocabulary size. Additionally, learners' word association types were analyzed based on the results of word association task. The results showed that Korean EFL learners' vocabulary size had significant correlations with their accuracy in identifying syntagmatic and paradigmatic associations, but not in the phonological association. However, their accuracy was not correlated with reaction times. The results indicate that L2 learners' mental lexicon is partially dependent on their vocabulary size of the target language, and it is variable depending on word association types.