The study aimed at investigating the effect of using A Differentiated Instruction Based Program to improve EFL Preparatory Stage Students' Reading Comprehension Skills and Self-Efficacy. The study adopted the quasi-experimental design. The participants of the study were (forty) second year preparatory School Students from Ahmed Saad preparatory school, Met Ghamr , Al Daqahlia Governorate, Egypt. The researcher utilized different instruments to collect data as, a reading comprehension skills checklist, a reading comprehension skills test and a Self-efficacy scale. The data obtained were statistically treated through the SPSS program. The findings of the study revealed that the program based on differentiated instruction had a positive effect on developing second year preparatory students' reading comprehension skills and self-efficacy.