The current study aimed to reveal the effectiveness of using the Elaboration theory to develop achievement in Arabic grammar among preparatory school students. The researcher chose as the research sample a group of second-year middle school students consisting of (sixty-four) students at the (Maharaza Boys Preparatory Institute) affiliated with the Abu Tasht Al-Azhar administration. Qena Governorate, and they were divided into two groups: one experimental, the other control, and the measurement tools consisted of an achievement test with the aim of measuring the second-year middle school students' achievement of the information and knowledge included in the unit “Positives of Nouns" in the Arabic grammar subject scheduled for the second semester of the academic year (2021/2022). It was used as a pre-test to measure the students' previous knowledge of grammar, and it was also used as a post-test to measure what the students gained through studying the unit using the steps of the elaboration theory. The research found that there were statistically significant differences between the average scores of the students in the experimental group and the scores of the students in the experimental group. The control group in the post-application of the achievement test was in favor of the experimental group, and there were statistically significant differences between the average scores of the experimental group students in the pre- and post-applications of the achievement test in favor of the post-application.