The Formative Assessment (FA) is a result of formative pedagogy and intends to open new paths for education, being a teaching conception, which considers that learning is a long process where the student restructures their knowledge from the activities they perform. Science Fairs (SF) can be an ally in FA in schools because they are considered non-formal educational activities with the purpose of promoting development of scientific culture. The present study aimed to analyze the effects of FA on a SF with elementary school students in the final years. It started from a documented analysis about the students' path during the project entitled as SF. The FA is evident through many steps of the process. The researched project was formative as it helped students learn how to develop themselves by participating in the learning regulations and development towards an educational project within the school.