This study was conducted to determine the level of student teachers' understandings of Newton's laws of motion and relating these levels to identify student teachers' models of understanding. An achievement test composed of two parts comprising 12 open ended questions was constructed and given to 45 pre-service classroom teachers. The first part of the test included 3 open-ended questions for each law asking students to explain a case, determine the physics law related to the case and write a case suitable for the related law. The second part was composed of three questions about defining Newton's laws of motion. The data analysis was carried out in two stages. In the first stage, the analysis of understanding level showed that pre-service teachers were relatively successful at explaining a case about Newton's law of motion. However, their achievement in defining these laws was low. In the second stage, the analysis of the understanding model revealed that students developed different understanding models such as Optimum Model (OM), Uncreative Model (UM), Theoretical Model (TM), Practical Model (PM), Memorizing Model (MM) and Inappropriate Model (IM). As a result of the data analysis it was determined that only a few students had a scientific understanding model. The results showed that the student teachers' have significant weaknesses in understanding the terms of fundamental knowledge of Newton's Laws of Motion. This may stem from the lack of student teachers to relate scientific knowledge with real life phenomena and experiences. Another result of this study is that defining the level of understanding and model of understanding students can help educators to prepare and implement teaching activities more effectively to promote students' thinking, discussing and interpreting skills. (Contains 5 tables.)