This research aimed to develop a natural of biology learning materials using guided inquiry model the validity, practical, and effective to facilitate student SMAN 1 Alas critical thinking skill. The learning instrument is developed by using 4-D model with a trial design one group pretest posttest design. Learning materials involving 30 students measure applicability of the learning plant, activities, the result of students learning in from of the critical thinking students and response to the deployed learning plant. The data collection apply data validation, observation, test, and questionaries. The researh date were analysis descriptively both quantitative. The validity of material syllabus, learning plans, studen book, student worksheet, and a valid critical thinking tes instrument. The practicality of the learning instrument that can be foud from the implimentation of the lesson plans that is categorized as good, and activities during the learning proses. The effectivenes of the learning materials that was reviewed through the results students critical thinkingability measured with n-gain reach high level of category and students highly excellent response towords the instrument and implementation of learning. It can be concluded that the biology learning instrument by using guided inquiry to fascilitate critical thinking skill.