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This study aims to improve understanding of the concept of Mathematics in class II of Elementary School Maguwoharjo 1 by using the Problem BasedLearning. In this study using the classrooom action research method using a cyclesystem consisting of planning, implementation, action, observation and reflection.The assesment used in this study is a test technique to determine understanding ofmathematical concepts. The resultts of the study showed that there was an increasein the percentage of the assesment of understaanding of student learning outcomes.The subjects of this were class II students at Elementary School Maguwoharjo 1Sleman Regency. The increase in understanding of concepts in learnngmathematica was know by the test resulth in Cycle i and Cycle II which showed anincrease in the average value and percentageo of classical completeness. Theaverage score of students learning Mathematicsin the initial conditions (pre-cycle)was 16 with classical completeness of 36% with a total of 8 students. Cycle onewas 72 with classical aptitude of 57% with a total of 15 students. Cycle II was 81with classical completeness 86% with 25.Keywords: Concept understanding, Math learning, Problem Based Learning