Abstract. Science process skills not only a learning approach but also as a result of learning. It is a basic skills in science and a tools of scientist to investigate the science phenomena. The focus of this research is to develop a valid and reliable of Science Process Skills Instrument (I-KPS). This research is Research and Development (R&D). The step of this research is theoretical construction, determination of assessment objectives, construction of items indicator, items construction, experts judgment, revising of instrument, the first field trial, revising of instrument, the second field trial, and finalization of instrument. The subject of field trail is 46 student of grade 10 of High School of 1 Bajeng, Gowa Regency. The instrument (I-KPS) specification are: (1) the type of item is essay test completed by scoring rubrics, (2) I-KPS could be assess science process skill for high school student, and (3) purpose of this I-KPS is cognitive test of science process skill. It is developed for measuring 6 (six) indicators of science process skills that is identification variable, hypothesizing, planning the experiment, predicting, communicating, and interpreting data. The content validity coefficient of I-KPS is 0.96. The result of empirical validation is 45 items are valid in topics of newton law, gravitational force, work and energy, momentum and impulse, and harmonic motion. The reliability coefficient of I-KPS is 0.935. The conclusion is I-KPS as valid and reliable instrument both theoretically and empirically.