The present study aims to measure the effect of the Roundhouse Diagram strategy on grainmar achievement and retention of this achievement for grade eight students in Cycle Two in Sultanate of Oman. The researcher prepared a special booklet that contained guidelines on how to improve students' achievement using the Roundhouse Diagram strategy as well as a grammar achievement test. Both the test and the booklet were validated by a specialist jury. The reliability of the test was found to be 0.82 using Cronbach's alpha. The sample of the study included two groups: The experimental group which had 34 students who studied Arabic grammar using the Roundhouse Diagram strategy and a control group that had 30 students who studied grammar in the regular conventional way. A post test was administered to both groups right after the experiment to assess students' grammar achievement and two weeks later to assess retention. Findings of the study reveal that there was a statistically significant difference between students' inean scores in both post-tests (the one used to measure achievement and the one used to measure retention) in favour of the experimental group. Based on these findings, the researcher makes a number of recommendations. One of these is that it is important to train Arabic teachers on how to use the Round House Diagram strategy not only to teach Arabic grammar but also other branches of the subject such as rhetoric.