SUMMARY
Vocabulary knowledge is the building block of learning a second language and the degree of success for learning any language depends on the amount of vocabulary a learner possesses. Vocabulary knowledge contributes to mastering language skills (reading, listening, writing, and speaking). Therefore, the present study aims at determining the role of vocabulary size in reading comprehension among Saudi EFL learners. The participants of this study included 64 male students who studied at the first level in the academic year 2018/2019 at King Khalid University. Both the Vocabulary Size Test developed by (Schmitt et al.) and reading comprehension test taken from the TOEFL preparation manual were used to collect the necessary data for the study. The results of the study revealed that the overall vocabulary size of Saudi EFL learners was 2025 word families. This amount helps students to comprehend 90% of written texts as pointed out by many researchers in this field. The results also showed a significant relationship between vocabulary size and reading comprehension. Vocabulary knowledge is an important predictor for comprehending written texts. The study provides some implications for educational stakeholders such as putting more emphasis on teaching vocabulary and using different teaching strategies that assist in the acquisition of vocabulary in general and academic vocabulary in particular. Keywords: vocabulary knowledge; vocabulary depth; vocabulary breadth; reading comprehension; Saudi EFL learners.