The aim of this study is to investigate the impact of lexical diversity on writing proficiency of Saudi EFL learners. Seventy-five Saudi EFL learners at the tertiary stage took part in this study (39 in level 2 and 36 in level 7). Their writing work is assessed and analysed using "Text Inspector" (a web-based language analysis tool). The results show that the lexical diversity of the participants lies within the normal range of second language learners. The findings also show a significant positive correlation between lexical diversity and writing proficiency. However, significant differences in lexical diversity or writing proficiency attributable to different academic levels could not be established. These results suggest that attention should be paid to vocabulary, as it is crucial to language learning. However, further research is needed to investigate the impact of lexical diversity on writing skill using different measures of lexical diversity.