This research aims to develop a Natural Science Learning Material based on Guided Inquiry Model that is feasible to improve students’ critical thinking skill. The learning material is developed  using 4-D model through a try out-design namely one group pre-test - post-test design to 12 students’ of VIII-2 class at SMPN 3 Tanjungbalai in the second semester of 2016/2017 academic year. The research findings were as follows: (1) The validity of the learning instrument which comprises Syllabus, Lesson plans, Students` book, Students` worksheet, and a very valid critical thinking test instrument; (2) The practicality of the learning instrument that were taken from: (a) the implementation of the lesson plans that is in good category (b) students activities were appropriate to the steps of guided inquiry and (3) The effectiveness of the learning instrument that was reviewed through: (a) the students’ critical thinking ability measured by n-gain in moderate level (N-Gain; 0,67 with criteria middle), (b) students’ highly positive response towards the material and the learning process. Based on the data analysis, it can be concluded that the Natural Science Learning Material based on guided inquiry model was feasible to improve students’ critical thinking skill