Concept mapping is a technique that paves the way to represent knowledge schematically. In this research, concept mapping was used as an assessment method on the impulse-momentum topic. The purpose of this study was to determine teacher candidates' knowledge about understanding of the concepts of impulse and momentum by comparing and contrasting two different methods; namely, students' concept maps and an achievement test. The mean of teacher candidates' concept map scores are extremely low when compared with the scores of the achievement test. In addition, it was seen that although a great number of concepts were written down, not many relationships were established between these concepts. There is a weak correlation between the achievement test and the concept map scores since concept maps assess the students' knowledge from a conceptual perspective while the achievement tests measure the level of students' knowledge on the topic and his/her ability to apply this knowledge on different occasions.