Penelitian ini bertujuan untuk mengetahui jenis-jenis kesalahan yang dilakukan oleh siswa berkemampuan awal matematika tinggi dan kemampuan awal matematika rendah dalam menyelesaikan soal geometri berbasis TIMSS untuk domain kognitif mengaplikasikan (applying) dan penalaran (reasoning). Jenis penelitian ini adalah penelitian kualitatif dengan menggunakan pendekatan deskriptif. Subjek penelitian sebanyak 2 orang dengan 1 orang berkemampuan awal matematika tinggi dan 1 orang berkemampuan awal matematika rendah dari siswa kelas VIII SMP. Instrumen penelitian menggunakan terdiri atas tes matematika berbasis TIMSS yang dikhususkan pada konten geometri domain kognitif aplikasi dan penalaran serta pedoman wawancara. Kesalaan yang dianalisis dikategorikan menggunakan kategori kesalahan Newman. Hasil penelitian menunjukkan bahwa: (1) Untuk kategori soal mengaplikasikan (applying), siswa kemmpuan awal matematika tinggi tidak mengalami kesalahan, sedangkan siswa berkemampuan matematika rendah mengalami jenis kesalahan transformation, process skills, dan encoding. (2) Untuk kategori soal penalaran (reasoning), siswa berkemampuan awal tinggi mengalami kesalahan tipe transformation, process skills, dan encoding sedangkan siswa berkemampuan awal rendah mengalami kesalahan tipe comprehension, transformation, process skills, dan encoding. Kata kunci : Kesalahan, Soal Matematika TIMSS, Domain Kognitif applying dan reasoning konten geometri, Kemampuan Awal, Kriteria Newman. This study aims to know the types of errors committed by students with high initial mathematical abilities and low initial mathematical abilities in solving TIMSS-based geometric questions for the cognitive domain of applying and reasoning. The type of this research is qualitative research using a descriptive approach. The research subjects as many as 2 people with of 1 people with early mathematical abilities and 1 people with low initial mathematical abilities from class VIII SMP. The research instrument consists of a TIMSS-based mathematical test devoted to the geometry content of the cognitive domain application and reasoning and interview guidelines. The difficulties analyzed were categorized using the Newman error category. The results of this research showed that: (1) For the category of applying , students with high initial mathematical abilities did not have errors, while students with low mathematical abilities had types of transformation errors, process skills, and encoding. (2) For the category of reasoning, the errors type of the students with high initial mathematical abilities are transformation, process skills, and encoding while students with low mathematical abilities had comprehension, transformation, process skills, and encoding errors. Keywords: Errors, TIMSS Mathematical Questions, Cognitive Domain applying and reasoning content geometry, Initial Capability, Newman’s Criteria.