The purpose of the study was to examine the impact of Differentiated Instruction (DI) on English writing fluency of the First Year Secondary Al-Azhar Students. To answer the study questions, the researcher adopted the quasi-experimental approach (One-Group Design). The study participants from Al-Azhar Secondary Stage students in Sohag Governorate. A program was used in teaching the study group in the second term of the year (2018-2019). A writing Fluency test was designed and validated to be used as a pre-posttest. The data of the study were analyzed, using Statistical Package for Social Science (SPSS 19), to confirm the test validity and reliability. On the other hand, t-test was used to measure the statistical differences in mean scores between the pre and posttest administrations of the test mean scores due to the use of the program. The study indicated that there are statistically significant differences at the level of (0.05) on English writing fluency skills of the first year secondary students in favor of the administration of the posttest. It means that the use of the program had a significant impact on the students. Based on those findings, the study recommended implementing the suggested program for developing writing to bring about better outcomes in students' writing fluency. It also was suggested that further research should be conducted in relation to other language domains.