In this study, the effect of open inquiry based learning (OIBL) approach on students' conceptual understanding was investigated. Quasi-experimental research design with pre-test and post-test control groups was used as research design. The research sample consisted of sixty students in 7th grade. Thirty students in the sample were in the experimental group, thirty students were in the control group and the experimental and control groups were randomly assigned. Liquid Pressure Conceptual Understanding Test (LPCUT) was used to determine the students' ideas about liquid pressure. Semi-structured interviews were conducted with nine students in the experimental group before and after the instruction. A rubric consisting of five categories was used in the analysis of LPCUT. According to the results of the study, it was determined that the students in the experimental group who received instruction based on open inquiry gave a higher rate of scientific answers than the students in the control group. Based on the results of the research, it is thought that the use of the instructions revealed in the OIBL environment in science courses may be beneficial for the academic achievement of the students. Moreover, an increase was observed in the scientific response rates of the students in the group taught according to the science program. In the light of the results obtained from this study, it can be suggested to investigate the effects of OIBL on students' ideas in different concepts.