This study investigated the effectiveness of inquiry-based learning (IBL) approach in ratio and proportion on the mathematics reasoning skill of seventh-grade students. The study was carried out in a seventh-grade mathematics course in a middle school located in the Central Anatolia region of Turkey during the 2016-2017 academic year. The IBL content was prepared and implemented about the ratio and proportion topics on which the reasoning skill is effective in the 7th grade curriculum. The IBL teaching implementations were conducted with 30 seventh grade students, but nine students, who represented different math achievement levels, were selected for the study's analysis. Course video recordings, worksheets, student interviews, and diaries were used as data collection tools. The results showed that the students' predictive, explanation, generalization and justification skills emerged as indicators of reasoning skill. Students made different predictions and generalizations based on their existing knowledge and they developed solutions to problems using different strategies in IBL process. According to these findings, it was concluded that students' reasoning skill were effective during IBL.