The aim of this study is to know the effectiveness of mastery learning strategy in the unit of "Integers and the operations on them" and comparing it with the prevailing teaching style in the achievement and retention of learning impact in the seventh (71) grade students. The study community consisted of all (7) grade students enrolled in the public schools under the Directorate General of Education in Muscat governorate, for the academic year 2012/2013. A sample of (61) students was selected from "Baraka bint Thalaba basic Education School (5-7)". The sample was split into (2) groups: one is experimental group with (29) students learning the unit of (integers and the operations on them) in accordance to the mastery learning strategy, and the other one is control group with (32) students learning the same unit using the normal class teaching method. The study tools contained of a set of formative tests covering all the (5) lessons included in the unit of integers and the operations on them) with a set of remedial and enrichment activities to be used in the experimental group only, in addition to the achievement test posttest at the end of the teaching unit, that measures the (3) cognitive levels: (remembering - understanding - implementation) and conducted on both groups. Data were then analyzed using T-Test for (2) independent samples to measure the differences and compare the performance of the experimental and control groups on the variables of the study. The results showed that: there were statistical significant differences between the achievement of both the experimental and the control students groups in the final achievement test posttest and the retention of learning impact in the unit of (integers and integer operations), and the achievement of the experimental group was the best. This confirms that the textural tests in teaching mathematics lead to substantial improvement in students achievements and the retain of the information for a longer period, and these results are attributed to the teaching method of the experimental group. Based on the above study, the researcher recommends to improve the prevailing teaching style to enhance student's achievements and their strong-points and to eliminate their weaknesses. It is also recommended that the mathematics study guideline shall include the procedures that the students need to follow at the end of each unit in order for the teacher and students to smoothly move to the next unit as per the mastery learning strategy.