The aim of this research is to investigate the effect of open inquiry based learning on the development of 7th grade students' academic achievement and science process skills. At the same time, the practicability of open inquiry in the science classroom and problems encountered by the teacher were examined. Mixed-method was used regarding the aims of the study. In the quantitative part, quasi-experimental study was designed whereas semi-structured interview was conducted with the teacher after the treatment in the qualitative part. The sample of this study was composed of randomly selected 2 classes from a conveniently selected public school with 7 classes. Open inquiry based learning activities developed by the researchers were conducted in the experimental group (n= 33 students) by their science teacher. In the control group (n=32 students), lessons were conducted as suggested in the science curriculum by the same teacher. Science process skills test and an achievement test about "Force and Motion" unit were used to collect the quantitative data. The results of MANCOVA indicated statistically significant differences on behalf of the experimental group. The teacher was unsure about when and to what extent she should have given scaffolding and how she should have guided the students during the open inquiry process. In line with the research results, the use of open inquiry based activities to improve students' academic achievement and science process skills is recommended.