Teachers sometimes fail to give varied teaching instructions that are suitable for a mixed-ability classroom because these instructions do not match students' proficiency levels. The current quasi-experimental quantitative study investigated the effectiveness of differentiated instruction strategies on students' overall English achievement. 60 grade 8 students from 2 different randomly selected schools from Irbid, in Jordan participated in this study. The experimental group (N=30) were taught English following differentiated instruction of homogeneous grouping, tiered assignments and tiered instruction in the areas of content, process and product of the Grade 8 text books with supplementary materials and differentiated reading texts. However, the control group (N=30) were taught english from grade 8 Jordanian English text book, Action Pack following the one size-fits-all method. Following the pre-test/post-test quantitative design, t-test results indicated that there was a statistical significant difference between the experimental and the control group in favour of the experimental group. Moreover, the results showed that differentiated instruction had a sizable effect size in reduction classroom diversity. The researcher recommended implementing differentiated instruction over a longer period and on a larger sample.