Abstrak: Instrumen kognitif keterampilan proses sains siswa masih belum tersedia dengan baik sehingga guru masih kesulitan dalam menilai kemampuan keterampilan proses sains siswa dikarenakan harus memerlukan waktu yang lama untuk mengamati siswa satu persatu melalui lembar observer. Untuk itu dilakukan pengembangan instrumen kognitif keterampilan proses sains siswa yang bertujuan untuk mendeskripsikan (1) validitas instrumen kognitif, (2) reliabilitas instrumen kognitif, dan (3) tingkat kesukaran butir soal instrumen kognitif. Penelitian ini merupakan penelitian pengembangan yang mengacu pada model pengembangan Borg and Gall. Teknik analisis data berupa validasi instrumen kognitif, reliabilitas instrumen kognitif, dan tingkat kesukaran butir soal instrumen kognitif, menggunakan analisis program Rasch. Hasil penelitian menunjukkan: (1) Validitas instrumen yang dikembangkan tergolong dalam kategori valid (2) Reliabilitas instrumen yang dikembangkan tergolong istimewa, dan (3) Tingkat kesukaran butir soal instrumen yang dikembangkan terbagi dalam empat tingkatan yaitu 2 butir soal kategori sangat sulit, 2 butir soal kategori sulit, 4 butir soal kategori mudah, dan 1 butir soal kategori sangat mudah. Diperoleh simpulan bahwa instrumen kognitif keterampilan proses sains yang dikembangkan berkualitas digunakan dalam evaluasi pembelajaran. Kata kunci: Instrumen Kognitif, Keterampilan Proses Sains Abstract: Cognitive instrument of students’ science process skill was not available yet so that teachers still have difficulties in assessing students’ science process skill because it takes a lot of time to observe the students one by one using observation sheet. Therefore, the development of cognitive instrument of students’ science process skill is done which aims to describe (1) the cognitive instrument’s validity, (2) the cognitive instrument’s reliability, and (3) the cognitive instrument items’ difficulty level. This research is a development research which refers to Borg and Gall’s development model. Data analysis technique is cognitive instrument’s validity, reliability, and items’ difficulty level using Rasch’s program analysis. Research result shows that: (1)The developed instrument validity is categorized as valid (2) The developed instrument reliability is categorized as outstanding, and (3) The instrument items’ difficulty levels are divided into four levels, which are 2 items are very difficult, 2 items are difficult, 4 items are categorized as easy, and 1 item is very easy. It can be concluded that the cognitive instrument of students’ science process skill is qualified to be used in learning evaluation. Keywords: cognitive instrument, science process skill