The aim of this study was to investigate the relationship between students’ reasoning ability, learning approach and conceptual understanding of heat temperature and internal energy concepts. As instruments of this study, the Test of Logical Thinking Ability, the Learning Approach Questionnaire and the Three Tier Heat, Temperature and Internal Energy Diagnostic Test were used. All instruments were administered to 116 eleventh and twelfth grade students in a public school. Descriptive statistics and a step wise multiple regression analysis were computed to analyze data gathered with those instruments. According to the results of this study, students’ reasoning ability was found as the best predictor of their conceptual understanding on the heat temperature and internal energy concepts.  Students’ reasoning ability and the meaningful learning approach predict the 51% of the variance related to the students’ understanding on Heat, Temperature and Internal Energy concepts.  Results of this study showed the importance of students’ reasoning ability and meaningful learning approach on their understanding of heat temperature and internal energy concepts.