Background: Previous studies have been reported that when instrument assisted soft tissue mobilization (IASTM) and the self-myofascial release technique were used on the muscles. However, studies that applied the IASTM and self-stretching to the gastrocnemius muscle are thought to be necessary but there is no such previous study. Objectives: To investigate the effects of IASTM and self-stretching on gastrocnemius muscle thickness and the range of motion of joint in dorsiflexion in healthy college student. Design: Quasi-experimental design (single blind). Methods: The subjects were healthy college students in their 20s with a healthy body. As for the experimental method in this study, comparison between before and after the experiment was performed to compare the effects of myofascial release using IASTM and stretching. The preliminary survey investigated the range of motion (ROM) of ankle joint of the subjects. The thicknesses of gastrocnemius muscles were measured using ultrasonography. One day after the preliminary survey, IASTM interventions and self-stretching interventions were randomly selected. If IASTM intervention is selected, the IASTM of the gastrocnemius muscle was applied for 5 minutes. After than, muscle thickness and the ankle dorsiflexion ROM were measured. Subjects were asked to take a break for about one day after performing the intervention. Self-stretching was applied to the gastrocnemius muscle for 5 minutes identically. After than, muscle thickness and the ankle dorsiflexion ROM were measured. Results: The thickness of the gastrocnemius muscle decreased significantly IASTM intervention, and the ankle dorsiflexion ROM increased significantly IASTM intervention. Ankle dorsiflexion ROM increased significantly the selfstretching intervention. The amounts of change in ankle dorsiflexion ROM through the IASTM was significantly greater than that through self-stretching. Conclusion: In order to immediately increase muscle flexibility in a short time, the IASTM is more effective although the self-stretching method is also effective.