The purpose of this article is to validate the relevance of a concept inventory on fractions by measuring the presence and evolution of misconceptions among prospective primary and pre-school teachers, including the overcoming of their misconceptions during and at the end of the instructional intervention. Seven text statements were defined and composed a "Likert scale" concept inventory. This was administered to students at three different stages of the learning process to measure the understanding gain: before the start of the course, after the instructional intervention consisting of a lecture and a seminar on fractions, and finally at the end of the semester. The results from the initial testing confirmed the misconceptions about fractions of the students. Based on the experiment, it is thus obvious the need to allocate a longer time for understanding and fixing the concept of fraction for prospective primary and preschool teachers.