Most principles and propositions in the science of reading are derived from research on Latin orthographies, usually, in English while much less is known about Semitic orthographies, namely, Arabic. This study investigated the effect of vowels and type of genre on oral accuracy, oral rate, and oral comprehension in reading Arabic orthography. A convenience sample of 85 children (34 fifth male graders and 51 tenth male graders) was selected from two public schools in Saudi Arabia. The researcher developed two reading measures; the Fifth Grade Reading Measure and Tenth Grade Reading Measure. Each measure has two genres (informational and poetic) and two versions (shallow/vowelized and deep/unvowelized). Each child individually completed the two versions of the measure in his grade. The results revealed that the students read the shallow genres (informational and poetic) more accurately and with more comprehension but less rapidly than reading the deep genres. In addition, the students read the informational genre (shallow and deep) more accurately, rapidly, and with more comprehension than the poetic genre (shallow and deep). The discussion concludes that a) the nature of Arabic orthography, mainly vowels, is an indispensible variable to the literature of science of reading, b) oral reading accuracy, oral reading rate and oral reading comprehension are affected by the unique characteristics of the genre, and c) vowels in Arabic are important to improve oral reading accuracy, and oral reading comprehension for the first grades in primary school and later grades in secondary school as well.