The research was intended to know whether a free physics experiment could (1) improve student s knowledge and (2) erase student s misconceptions on thermophysics concepts about water. The sample of the research was 43 students of Physics Education Departement Sanata Dharma University. The research used a free experiment as a treatment. A free experiment is an experiment that allows student to decide prepare and do their experiment freely. Students are able to think and do experiment more constructivistically. The experiment s instruction is very simple and short only consisted of some questions that have to be answered by students and simple direction for student to decide plan do their experiment and make conclusion. The research used pretest and posttest as instruments. The tests were analyzed using dependent t-test to know the improvement of student s knowledge and using coding methods to know student s misconceptions and whether their misconception were elimated using the free experiment method. The result were (1) student s knowledge on thermophysics about water was improved and (2) student improved their misconceptions by having more fully concepts and by eliminating their misconceptions. So the free experiments could be used as one tool to improve student s misconception on physics. In addition the research gives one more example for research on physics education in Indonesia.