Virtual experiments have been developed on the concept of thermodynamics. This study was intended to analyze the level of understanding of students' thermodynamic concepts after learning with virtual labs. Model testing used the pretest-posttest control group design. The research subjects were fifty-four students divided into two groups, experimental and control groups. An essay test was given to test the level of conceptual understanding of students. Data was processed to examine differences in improvement in both groups. The results showed an increase in mastery of students' thermodynamic concepts in both groups. The increase in mastery of the thermodynamic concept in the experimental group was higher than the control group. However, statistical analysis showed that the differences between the two groups were not significantly different. The concept of temperature and expansion is the concept with the highest increase.