Providing ESL/EFL writers with freedom to choose a writing topic has been regarded as important for promoting their learning motivation and improving their quality of writing. Lexical diversity is one of the critical elements in evaluating writing quality, and several factors such as major/content knowledge, age, gender, or language proficiency influence the overall quality of writing even when L2 learners freely choose their writing topics. This study explored the relationships between overall language proficiency and lexical diversity, and between writing genres and lexical diversity when Korean EFL writers had freedom to choose their topics. Seventy-two university students participated in this study, and they submitted two essays of different writing genres during one semester. Their writings were analyzed in terms of the number of types, tokens, TTR, and VocD using a computer program, CLAN. It was found that VocD values differed depending on writing genres, but overall proficiency did not contribute to the differences. The length of essays was not related to the level of VocD either. The findings suggest that in order to improve the lexical diversity of EFL writers, educators need to provide them with diverse writing activities that include different types of writing genres.