The aim of this study is to determine the effect of formative assessment based learning on students' conceptual understanding levels regarding basic astronomical concepts. The study was conducted in a public primary school with 24 5th graders. The study group was applied a 3 phased teaching process based on formative assessment for the duration of 4 lesson periods. The conceptual understanding levels of students were specified by using 4 formative assessment probes prior to and after the process. Findings were presented and assessed in forms of percentage and frequency together with sample student answers. Prior to the teaching process it was determined that conceptual understanding levels of students regarding basic astronomy were rather low and students had developed various misconceptions. The reason for formation of day and night "is the exchange of location of the Sun and the Moon," the reason for lunar phases "is the Moon being a light source and radiating light from its different parts in different periods," the reason for different seasons to emerge "is the distance change of Earth from the Sun, in terms of coming closer and moving away" were some frequently observed misconceptions. After the activities the conceptual understanding levels of students showed increase. It was, nevertheless, challenging for them to explain their correct answers. This indicates the necessity of more studies to be conducted in order to develop scientific reasoning skills of students. This study recommends teachers to use formative assessment based education, which proved to be effective in teaching basic astronomical concepts, in science classes actively.