The aims of this study is measuring the diagnostic accuracy for curriculum based measurement (CBM) of oral reading fluency. The study focuses on screening students who are at the risk of reading disabilities in grade fourth in Muscat. Additionally, the study determines the classification of students' levels on oral reading fluency and to achieve these goals, the CBM were used. It is three equivalent tests in different formats and each test has three reading texts. The second used tool in the study is Learning Disabilities Diagnostic Inventory (LDDI). It was implemented as an instrument for teachers to determinate diagnostic accuracy. The CBM was applied on an pilot sample of 40 students, it helped to measure the reliability ofCBM. After that, the CBM was applied on the main sample consisting of 151 students. The results of this study shows the decimal point for the 25 percentile in determining students who are at the risk of reading disabilities is 40 words in one minute. It also exhibits that Receiver Operator Characteristic ROC keeps away toward the far left which indicates the diagnostic power of CBM in oral reading fluency during one minute to determine the risk of reading disabilities. Finally, the study found by useding chi-square that no difference based on gander to determine the three classification levels: frustration, instructional and mastery.