This research is motivated by the low mathematical critical thinking ability (KBM) of junior high school students which is estimated to be because the learning process does not provide opportunities for students to improve these abilities. One learning model that can be used to improve these abilities is Situation-Based Learning (SBL). The purpose of this study is to examine the increase in student KBM as a result of SBL learning. This research is a quasi-experiment that applies two learning models namely SBL and conventional learning with a population of all VIII grade students of one of the State Junior High Schools in Sumedang 2015/2016 Academic Year. Sampling was done by purposive sampling and obtained class VIII-E as an experimental class or class that received SBL and class VIII-F as a control class or class that received conventional learning. The research instrument used was the Early Mathematical Ability Test (KMA), the KBM test, and the observation sheet. Data analysis using the Mann-Whitney test and t-test. Based on the results of the data analysis, it can be concluded that: 1) the increase in KBM of students who received SBL was significantly higher than those who received conventional learning; 2) a. the increase in KBM for students who received SBL was significantly higher than those who received conventional learning in terms of high KMA; b. improvement of KBM for students who received SBL was significantly higher than those who received conventional learning in terms of medium KMA; c. Improvement of KBM for students who received SBL was significantly higher than those who received conventional learning in terms of low KMA