Much research has been done on the application of concept maps as means for learning assessment. Similarly, different types of Concept-Map Based Assessments (CMBA) and their validity have been examined as well in many studies, but to a far less extent. The present study uses a descriptive quantitative method and mainly aims to put forward the idea that the select-and-fill-in (SAFI) concept maps could be used as a valid instrument to assess the conceptual understanding of science among thermodynamics students. For this purpose, the concurrent validity of the SAFI concept map was evaluated according to the last version of the Thermodynamic Concept Survey (TCS) in order to develop a standard conceptual survey in thermodynamics. The TCS has a total KR-20 of approximately 0.78, an acceptable value, which could be employed as a valid test to assess teacher-made SAFI concept maps. The study population includes 60 students from two physics classes. An evaluation of the conceptual understandings of thermodynamics students were made concurrently using two assessment tools. Based on the study findings, there is a moderate to strong correlation (0.6) between the Teacher-made SAFI concept map and TCS. This leads us to the conclusion that SAFI concept maps are valid tools, at least, for evaluating conceptual understanding in thermodynamics. Moreover, the results of this study are confirmed a significant relationship between a student’s ability to read and comprehend a given question and his/her ability to solve it.