Berdasarkan observasi pretest yang dilakukan, siswa kelas XI-IPS kurang memahami konsep dari materi yang sedang dipelajari. Siswa kesulitan dalam menyatakan ulang sebuah konsep, mengklasifikasikan menurut sifat-sifat tertentu sesuai dengan konsepnya, menggunakan dan memanfaatkan serta memilih prosedur atau operasi tertentu dan mengaplikasikan konsep atau algoritma pada pemecahan masalah. Hal ini juga dilihat ketika siswa kesulitan mengerjakan soal�soal yang divariariasikan. Oleh karena itu, penelitian ini bertujuan untuk mengetahui apakah metode Guided Discovery Instruction dapat meningkatkan pemahaman konsep siswa kelas XI-IPS dalam belajar matematika dan bagaimana penerapan metode Guided Discovery Instruction dalam meningkatkan pemahaman konsep siswa XI-IPS dalam belajar matematika. Model penelitian yang digunakan adalah PTK (Penelitian Tindakan Kelas) desain Kemmis& Mc Taggart yang berlangsung selama dua siklus. Penelitian dilakukan di salah satu Sekolah Kristen Swasta di kota Ambon, Maluku dengan jumlah siswa 14 orang. Penelitian ini menggunakan instrumen berupa lembar tes, lembar angket, lembar umpan balik, lembar wawancara, lembar observasi, dan jurnal refleksi. Cara pengolahan data adalah analisis deskriptif kualitatif. Hasil penelitian menunjukkan bahwa metode Guided Discovery Instruction dapat meningkatkan pemahaman konsep siswa kelas XI-IPS dalam belajar matematika dengan pencapaian indikator yaitu pengenalan dan review, tahap terbuka, tahap konvergen, dan tahap penutup./ Based on the observation that was conducted during the pretest, most of the XI-IPS students had a difficulty to understand the concepts of the learning materials that were taught to reexplain a concept; classify objects according to the characteristic of a concept; use and choose a procedure or certain operation; and apply a concept or an algorithm in solving a problem. It was seen when the students were asked. The students also had a difficulty when they were asked to answer modified questions. Therefore, the aim of this research was to know whether Guided Discovery Instruction method can improve grade XI-IPS students, conceptual understanding in learning mathematics and also to know how can Guided Discovery Instruction method improve grade XI-IPS student’s conceptual understanding. The research method used was the Kemmis and Mc Taggart’s model of classroom action research, which lasted for two cycles. This research was conducted in a Private Christian Achool in Ambon, Mollucas. The number of the students in this research was 14 students test sheets, student questionare, mentor theacer’s feedback sheets, interview sheets, observation sheets, and the research’s by qualitative descriptive analysis. The result showed that Guided Discovery Instruction method could grade XI-IPS student’s conceptual understanding in learning mathematics through indicator’s, such as introduction and review, the open-ended phase, the convergent phase, closure.