Lexical diversity (LD) is an important indicator of second language lexical development. Much research has investigated LD indices, with a focus on learners of English. However, further research is needed in languages that are typologically distinct from English, such as Korean. In this study, we evaluated the reliability and validity of LD indices applied to argumentative writing produced by Korean learners. The results indicated that HD-D, MATTR, and MTLD were reliable across different text lengths and were correlated with holistic proficiency scores. However, the meaningful differences were found across Korean-specific tokenization types related to the way morphemes are processed.