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This research aims to determine the improvement of students' mathematical thinking abilities through the application of the guided discovery method for class VIII at UPT SPF SMP Negeri 39 Makassar. Thisresearch was conducted at UPT SPF SMP Negeri 39 Makassar using the classroom action research methodwhich was carried out in 2 cycles, where each cycle consisted of 4 meetings and 1 end-of-cycle test meeting.The subjects in this research were class VIII students at UPT SPF SMP Negeri 39 Makassar for the2022/2023 academic year, consisting of 24 students. The instruments used in this research were observationsheets of teacher and student activities during the learning process, learning outcome tests, anddocumentation. The research results show that the application of the guided discovery method. It canimprove students' mathematical thinking skills and can increase students' learning activities, this can beseen from the percentage of student activity in cycle I of 37.5%, which has increased in cycle II to 70.8%.Meanwhile, the increase in the percentage of ability to think mathematically in cycle I obtained a percentageof ability to question problems of 57.29%, an increase in cycle II to 72.92%, in cycle I the percentage ofability to test was obtained at 65.63%, an increase in cycle II to 65.63%. 73.96%, in cycle I the ability tojudge was 66.67%, increasing in cycle II to 81.52%, in cycle I the percentage of ability to make conclusionswas 60.42%, increasing in cycle II to 82.29%. So, the mathematical thinking ability of class VIII studentsat UPT SPF SMP Negeri 39 Makassar in cycle I was 62.42% and increased in cycle II to 72.60%. Basedon the research results, it can be concluded that the guided discovery method can be recommended as amethod that is able to improve the mathematical thinking abilities of class VIII UPT SPF SMP Negeri 39Makassar students.Keywords: Thinking Ability, Method, Guided Discovery, Mathematical Critical Thinking