The purpose of this study was to investigate the effectiveness of Interactive Lecture Demonstrations over traditional instruction on university students' understanding of heat and temperature. The participants were 327 first year undergraduate students from two science classes in two academic years from the same university in Thailand. One class was the experimental group taught using Interactive Lecture Demonstrations, while the other was the control group taught using traditional instructional methods. The Heat and Temperature Conceptual Evaluation test was administered to the experimental and control groups before and after instruction to evaluate student understanding. We found that the average normalized gain in the experimental group (g = 0.25) was higher than in the control group (g = 0.08). The results indicate that students taught using Interactive Lecture Demonstrations had a better understanding of heat and temperature concepts than students taught using traditional instruction. (Contains 3 tables and 2 figures.)