This study aims to analyze the effect of implementing the Inquiry Based Learning (IBL) model through the Lesson Study method on the Indonesian language learning outcomes of third-grade students at SDIT Al-Manar Pekanbaru. The research employed a quasi-experimental design with a quantitative approach. The subjects consisted of two classes: the experimental class, which was taught using the IBL model integrated with Lesson Study, and the control class, which received conventional instruction. Data were collected through learning outcome tests. The findings revealed a significant difference between the two groups, where the experimental class achieved higher average scores and better mastery levels compared to the control class. Therefore, it can be concluded that the implementation of the IBL model supported by Lesson Study is effective in improving Indonesian language learning outcomes of third-grade students at SDIT Al-Manar Pekanbaru.