Grammar mastery is one of the most important aspects of speaking skills. Weakness in mastering grammar is a phenomenon that often occurs in learning Arabic as a third language. Therefore, the purpose of this study is to identify the level of students' mastery of Arabic grammar and the level of mastery of Arabic speaking skills of diploma-level students who take Arabic as a third language courses at the University of Technology Malaysia (UiTM). In addition, this study will explain the implications of students' mastery of Arabic grammar on students' Arabic speaking skills. This study is a quantitative survey and the data is analyzed descriptively and inferentially. The study sample involved in this study was 350 people from a population of 1973 students. The research instrument used in this study consists of two sets of mastery test questions, namely the Arabic grammar mastery test (UTBA) and the speaking skill mastery test (UKBA). Through both mastery test scores, the researcher conducted parametric tests using the Pearson ‘r’ correlation test to test the relationship between the two variables. The results showed that the level of Arabic grammar mastery of UiTM diploma students is at a medium level. While the level of mastery of the students' Arabic speaking skills is at a very weak level. Meanwhile, the results of the Pearson ‘r’ correlation test on both variables showed that mastery of Arabic grammar and Arabic speaking skills have a highly significant relationship (r =0.508). It meant that when the mastery of the students' Arabic grammar increased, the mastery of the students' speaking skills increased at the same time. It is hoped that the results of this study will help students in improving their Arabic grammar proficiency and be able to apply it to Arabic speaking skills well.