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The Problem-Based Learning model is a strategy in implementing learning related to real problems in everyday life, the solution will be maximized if combined with Lesson Study for Learning Community (LSLC). The objectives of this study are: 1) to know the application of the Problem Based Learning (PBL) model based on Lesson Study for Learning Community (LSLC) to improve students' mathematical communication. 2) to know the result of the application of the Problem Based Learning (PBL) model based on Lesson Study for Learning Community (LSLC) to improve students' mathematical communication. This research is a type of Classroom Action Research (PTK) consisting of 2 cycles, each cycle consists of four stages, namely: planning, implementation, observation and reflection. The population in this study were all students of class VIIIA MTsN Karangasem totalling 32 students and the sample was taken 3 students based on the results of observations and test results, namely students with high, medium and low abilities. The instruments used in the research were observation, tests, interviews and documentation. Data analysis was carried out through three stages, namely, Data Reduction, Data Presentation, and Conclusing Drawing. The results of this study can be seen with the acquisition of the percentage of student activity and test questions increasing every cycle. In activity I cycle I increased 21.56% from 61.77% to 83.33% in cycle II, in activity II cycle I increased 19.10% from 69.10% to 88.20% in cycle II, in activity III cycle I increased 12.15% from 69.10% to 81.25% in cycle II. Likewise with the test results, in the first cycle test questions increased by 12.50% from 81.25% to 93.75% with an average value of the first cycle increasing by 6.28 from 78.75 to 85.03.