Due to the impact it has on a person's capacity to communicate in a foreign language, vocabulary mastery is a crucial component that must be taken into consideration. This study aims to determine the level of Arabic vocabulary mastery among students in Selangor, Malaysia's religious secondary schools (SAM), focusing on two aspects: vocabulary size and depth. This study was designed quantitatively by using a survey as a data gathering tool, which is a set of Arabic Vocabulary Assessment Tests (PekkA). The participants involved were a total of 335 Form Four students from Selangor State Secondary Religious schools. The data obtained was analysed descriptively using the Statistical Package for Social Science Windows (SPSS) Version 27. The results indicate that the students had a modest level of command of Arabic vocabulary (size and depth). Meanwhile, the students' degree of mastery Arabic vocabulary size was high, but their level of mastery its depth was moderate. This study shows that the instrument used has high validity based on its careful development procedure that includes aspects of size and depth based on the Common European of Reference for Language (CEFR) test concept. This study also makes a significant contribution to the importance of shaping student autonomy in developing good vocabulary learning ability.