This study aims to examine the effectiveness of science learning which applied Contextual Teaching and Learning (CTL) to improve the critical thinking skills of elementary school students on the topic of balance and preservation of natural resources. This research is a part of a development study with the main focus on the analysis of the effectiveness of learning aids using one group pre-test post-test. The study was conducted at SD Al Muslim, one of the primary schools (SD) in East Java, with a total sample of 80 students divided into three classes. Before the learning with CTL approach was carried out, students were given pre-test of Critical Thinking Skills (KPS) and after learning, students were again given the same KPS test as the final test (post-test). Data in the form of KPS skills, activities, and student responses were respectively collected using KPS test instrument, student activity observation sheet, and student response question. The collected data were then analyzed using a paired t-test, ANOVA, and descriptive analysis. Research has shown that CTL-based science learning is effective in improving students' critical thinking skills.