The present study aimed at investigating the effectiveness of a differentiated instruction-based program (DIBP) for developing English reading fluency among preparatory schoolers. To do so,  thirty students  were selected from  Fatima Al Zahraa  Prep School for Girls, Al-Arish, North Sinai Governorate. For determining the most important and required EFL reading  fluency skills for second year prep students, a list of EFL reading fluency  skills was designed and validated by a number of EFL jury members. Then,  pre and post EFL oral reading fluency test was designed. Students were pre-tested to determine their entry level of EFL reading fluency skills, then they were taught the program. Data were obtained from administering reading fluency post test. To analyze the data statistically, a paired sample t-test  was used. Results of the study revealed that the differentiated instruction based-program had a significantly positive effect on developing the reading   fluency skills in English for preparatory schoolers. Recommendations and suggestions for further research were presented.