This study investigates the relationship between vocabulary knowledge and reading comprehension of authentic Arabic texts; in particular, it attempts to investigate the percentage of vocabulary coverage (known words) readers need to ensure reading comprehension of two reading passages from online Arabic news source. Data was collected from twenty-three Arabic as-foreign language (AFL) learners at Brigham Young University, who ranged from Intermediate Low to Intermediate Mid in both productive and receptive skills. Two reading comprehension tests, circling the unknown words in texts and a lexical coverage test for each passage texts were given to the subjects. A linear regression analysis of the data shows that there is a correlation coefficient of 0.7 and 0.6 between the percentage of known words and students'comprehension of the two reading texts. The results indicate that the subjects needed to know approximately 90% of running words to adequately comprehend the first passage and around 86% to comprehend the second passage. Based on the findings, this study suggests that there is a lexical threshold for AFL learners,below which adequate comprehension of authentic texts might not be possible.