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Misconception is one of the issues that frequently arises in all areas of learning science. Mathematics is among the subjects with the greatest number of misunderstandings. This is because according to PISA 2019 data, the ability to understand an Indonesian Mathematics concept has decreased quite drastically.Another finding in class XII BDP SMK PGRI 13 Surabaya was that more than 50% of the students had difficulties understanding the ideas presented by the teacher as well as those found in books used as learning resources, particularly in this study that concentrated on the material of ascending function and function down. This study intends to discover what proportion of students have misconceptions and to investigate the origins of misconceptions in order to suggest alternative preventative strategies. The research design is descriptive qualitative, and the methods for collecting data are a three-tier written diagnostic test (the "three tier test"), two-stage interviews, and direct observation. According to the findings of this study, eighty percent of BDP class XII students had misunderstandings, which were classified as theoretical misconceptions, calculation misconceptions, and systematic misconceptions.