Vocabulary knowledge plays an important role in writing. Previous research has examined the relationship between various aspects of vocabulary knowledge and independent writing performance and limited attention has been given to integrated writing such as summary writing. Our study investigated the contribution of two aspects of vocabulary knowledge (vocabulary size and lexical richness) to summary writing quality. In addition, we examined the extent to which lexical richness measures relate to summary writing quality among students who have smaller vocabulary sizes and students who have larger vocabulary sizes. We addressed these questions by administering a lexical decision test called the LexTALE and analyzing lexical richness and writing quality in the summary writing of 73 English as a Foreign Language (EFL) learners of English. The results revealed a strong positive correlation between vocabulary size and summary writing quality indicating that a larger vocabulary size is associated with higher-quality summaries. However, most measures of lexical richness did not show a significant correlation with summary writing quality. Interestingly, the relationship between lexical richness measures and summary writing quality varied when learners were put in two groups with different vocabulary sizes. Implications of the findings of the study for language teaching pedagogy and for research on the complex relationship between vocabulary knowledge and summary writing are discussed.