Abstrak: Keterampilan proses sains siswa kelas X IPA 1 SMA Negeri 1 Marabahan masih tergolong rendah, hal ini diduga model pembelajaran yang diterapkan di sekolah belum mampu meningkatkan keterampilan proses sains siswa. Penelitian ini bertujuan meningkatkan keterampilan proses sains siswa kelas X IPA 1 SMA Negeri 1 Marabahan melalui pembelajaran guided inquiry. Tujuan khusus penelitian adalah mengetahui keterlaksanaan RPP, keterampilan proses sains, dan hasil belajar siswa setelah dilakukan pembelajaran dengan guided inquiry. Jenis penelitian menggunakan penelitian tindakan kelas. Teknik pengambilan data melalui observasi, tes hasil belajar, dan dokumentasi. Hasil penelitian dianalisis secara deskriptif kualitatif dan kuantitatif. Temuan penelitian yaitu: (1) keterlaksanaan RPP pada siklus I sebesar 3,71; siklus II sebesar 3,75; siklus III sebesar 3,87 dengan kategori sangat baik, (2) keterampilan proses sains siswa berkategori sangat baik, (3) hasil belajar siswa pada siklus I sebesar 79,17%; siklus II sebesar 91,67%; siklus III sebesar 100%. Berdasarkan temuan diperoleh simpulan bahwa model pembelajaran guided inquiry dapat meningkatkan keterampilan proses sains siswa kelas X IPA 1 SMA Negeri 1 Marabahan. Kata kunci: keterampilan proses sains, hasil belajar, guided inquiry, PTK. Abstract: The science process skill of class X-science of Marabahan Senior High School is still relatively low, it is suspected that school applied learning model not able to improve the science process skill. The purpose of this research is improve the science process skill of class X-science of Marabahan Senior High School utilize guided inquiry. The specific purpose is record feasibility the learning plans, science process skill, and student’s learning outcomes. Type of research is classroom action research. Collecting data through observation, learning outcomes test, and documentation. The results were analyzed by descriptive qualitative and quantitative. The findings of the research are: (1) feasibility of learning plans in the first cycle is 3,71; in the second cycle is 3,75; in the third cycle is 3,87 with a very good category, (2) science process skill is categorized very good, (3) student’s learning outcomes in the first cycle is 79,17%; in the second cycle is 91,67%; in the third cycle is 100%. Based on the findings obtained the conclusion that guided inquiry can improve the science process skill of class X-science of Marabahan Senior High School. Keywords: science process skill, student’s learning outcomes, guided inquiry, PTK.