The present study is an attempt to explore the relationship between the breadth (the number of words known) and the depth of vocabulary knowledge (the richness of word knowledge) (the richness of word knowledge), and reading comprehension of EFL learners in an English as a foreign language (EFL) context. Furthermore, it tries to find the effect of language proficiency level of learners on the breadth and the depth of lexical vocabulary and reading comprehension of Saudi EFL learners. The participants of the study were seventy-five intermediate and advanced level majoring in English Translation at Qassim University in KSA. The level of language proficiency of participants had been checked through an OPT by the department in advance. To collect the relevant data, two tests measuring breadth and depth of vocabulary knowledge were administered to all participants. They also received a reading comprehension test in which they were asked to read the passages and answer some multiple-choice questions. The findings obtained from the analysis of the data indicated that there is a significant relationship between breadth and depth of vocabulary knowledge and Saudi advanced/intermediate EFL learner's reading comprehension performance totally. The results further revealed that both dimensions of vocabulary knowledge are positively correlated, that is, those learners who had large vocabulary size had a deeper knowledge of the words, too. The results further showed that language proficiency level of learners have an effect on Saudi EFL learners' reading performance and vocabulary knowledge. These results confirm the importance and the value of developing students? breadth and depth of vocabulary knowledge in EFL classrooms.