This study is conducted to analyze the understanding of students' concepts in trigonometry courses based on the theory of APOS (action, process, object, and scheme). Based on previous research, students had difficulty understanding the concept of trigonometry, so they had difficulty applying the common trigonometric formula, explaining each one's relation to the comparative trigonometry, and performing calculations and algebra calculations. This type of research is qualitative research descriptive. The subject of the study is a student in the mathematics education semester for two academic years, 2023-2024. Randomly selected research samples using simple random sampling. Analyzing five student responses in the Trigonometry course reveals variations in conceptual understanding based on the APOS theory. Some students successfully followed the stages of action, process, object, and schema comprehensively, while others struggled to apply the correct concepts, particularly in solving problems using trigonometric principles. From this research, it can be concluded that it is important to understand the concept of trigonometry so that the subject can answer a question based on APOS theory