The current study investigated the effect of Arabic diacritics on rapid naming of fourth grade students with and without reading disabilities. The rapid naming subtests of the comprehensive test of phonological processing (CTOPP) and three tests of rapid naming of letters, words and passages were administered to (200) fourth grade students in school in Muscat. Participants included students with and without reading disabilities from both genders. The study findings showed that diacritics paly an important role of one of the two components of reading fluency in children with and without reading disabilities. Diacritics contribute to the accuracy of naming but hinder its speed. The study findings are discussed in relation to The Double Deficit Hypothesis of reading disabilities. Implications of the study include the importance of further investigations of Arabic orthography particularly deep orthrography which is void of diacritics in later stages of written language.