Perkembangan ilmu pengetahuan yang terjadi di seluruh aspek kehidupan saat ini, menjadikan Matematika sangat menonjol. Sementara itu, di lain pihak matematika termasuk mata pelajaran yang tidak disukai oleh siswa. Bagi mereka, matematika menjadi “momok” yang kalau bisa dihindari. Karena pendapat ini, banyak siswa yang kurang termotivasi belajar matematika sehingga terhambat untuk memahaminya. Untuk meningkatkan motivasi belajar siswa tersebut, diperlukan strategi yang bagus. Salah satu strategi yang dapat digunakan adalah penggunaan tes formatif. Penelitian ini menggunakan metode penelitian tindakan kelas model Kemmis & Mc Taggart, yang terdiri dari tiga siklus. Data penelitian diambil saat praktikum terakhir peneliti yang berlangsung di Lampung, dengan subjek penelitian peserta didik kelas X SMA. Data dikumpulkan dengan menggunakan jurnal refleksi dan umpan balik mentor, dan dianalisis menggunakan analisis deskriptif. Berdasarkan hasil penelitian diperoleh kesimpulan bahwa penggunaan tes formatif pada pembelajaran Matematika kelas X dapat meningkatkan motivasi belajar siswa. Berdasarkan jurnal refleksi peneliti diperoleh hasil bahwa motivasi siswa meningkat. / The development of knowledge occurs in every aspects of life nowadays caused Mathematics to be very influential. However, Mathematics remains the most disliked subject by students. They see Mathematics as the threat that has to be avoided. Because of this perspective, a lot of students are lack of motivation in learning Mathematics which later hinder their pace to understand it. To improve students’ motivation, there has to be a very good strategy. One of the strategy available is the use of formative assessment. This research applied Kemis & Mc Taggart’s class action research approach that consists of three cycles. The data was obtained when researcher was doing his last teaching practice in Lampung, and the research subjects were grade X Senior High School students. The data was gathered from reflection journals and feedback from mentors which were then analyzed descriptively. Based on the result of this research, the conclusion is that the use of formative assessment in learning Mathematics in grade 10 can help improve students’ learning motivation. Based on the information in researcher’s reflection journal, it shows that students’ motivation improved.