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The objective of this study was to observe the effect of Connecting, Organizing, Reflecting, and Extending (CORE) instruction model by work card toward students- mathematical problem-solving skill in Junior High School 1 Klari. The research method was quasi-experiment using non-equivalent post-test control design. All classes in seventh grade become the population. Two-stage sampling technique, namely purposive sampling and cluster random sampling technique, was used to select VII-A as the experimental class who treated with the CORE instruction model by work card and VII-B as the control class who treated with direct instruction model. Students- mathematical problem-solving skill was measured by the test which created based on problem-solving skill indicators. Based on hypothetical testing using independent sample t-test with a significance value of 0.05, the result has shown that there was a significant difference between the experimental and control class. There was t count = 4.6932 > t table = 1.667, then H0 was rejected. This result was indicated that the average of the students- problem-solving skill who using CORE instruction model by work card higher than the average of the students- problem-solving skill who using conventional instruction model. Therefore, it can be concluded that the CORE instruction model by work card affected to students- mathematical problem-solving skill in Junior High School 1 Klari.