This study reports the outcomes of research investigating the relationship between two variables; percentage of word knowledge in an academic English text and reading comprehension scores the participants got for the same text. The study was conducted at Prince Sattam Bin Abdulaziz University, Preparatory Year Deanship, during the first semester of 2017-2018. Seventy-one Saudi male students participated in the study. They came from four groups of students selected from the Preparatory Year population to represent four various levels of English (Groups one, seven, fourteen and seventeen) based on their achievement in the placement test. Data was collected using two instruments: two word-meaning association test-lists and a reading comprehension test. The results showed that there is a statistically significant correlation (r = 0.702) between percentage of known words and reading comprehension in general i.e., for all the participants. The effect of word knowledge on reading comprehension was very high R square = 0.49. However, results for the groups separately showed that correlation was positive but not statistically significant for the higher levels i.e., groups one and seven. Also, there was a low effect of word knowledge on comprehension. For the lower levels i.e., groups 14 and 17, the correlation was positive and statistically significant. Results also showed a very high effect of vocabulary knowledge on reading comprehension. Based on the results and analysis of the study, the researchers provided recommendations that will help improve reading curriculum selection and English teaching practices, and suggestions for deeper research into the topic that will include female students and other related concepts.