Biology lesson is one of the leading lessons that the students at all educational levels encounter with difficulty in grasping the subject matter and the teachers in teaching it. The reasons for this situation can be identified as the methods and techniques used in the education process being traditional, the fact that some biology course’s subject matter concepts are abstract and the teachers have difficulties in concretizing while teaching them and the students having difficulties in grasping these abstract concepts. The purpose of this study was to investigate the effect of the formative assessment method on the conceptual understanding of 7th grade students about the digestive system in the "systems in our body" unit in the science course. The study was carried out with 105 seventh grade students. Formative assessment probes were given to both the experimental and control groups as pre-test and post-test. Formative assessment strategies were developed and applied in an attempt to replace the existing misconceptions with scientific concepts. A mixed method was used in the study. The findings illustrated that there was a significant difference about the digestive system between the conceptual understanding levels of the experimental group students and the control group students.