Objective: This study was aimed to compare the thickness and pennation angle of gastrocnemius through ultrasonography during the heel-drop exercise on ankle dorsiflexion angle. Design: Cross-sectional study. Methods: Nineteen normal adults in their 20s had voluntarily participated in this study. All subjects performed the ankle heel-drop exercise with ankle dorsiflexed to 0°, 10°, and 20°: heel-drop exercise with ankle dorsiflexed to 0° was executed on floor-level, heel-drop exercise with ankle dorsiflexed to 10° on a wooden-block of 2.3 cm in height, and heel-drop exercise with ankle dorsiflexed to 20° on a wooden-block of 5.5 cm in height. In each regimen, the subjects completed a session of 100 heel-drop exercises (10 repetitions×10 sets; with 30 seconds rest following each set; with 24 hours rest following each exercise). Before and immediately after each heel-drop exercise, the thickness and pennation angle of gastrocnemius were measured using an ultrasonography. Results: After the performance of the heel drop exercises with ankle dorsiflexed to 0°, 10°, and 20°, the thickness of the gastrocnemius was significantly higher than pre-exercise (p<0.05), and furthermore heel-drop exercise with ankle dorsiflexed to 10° was significantly higher than exercise with the ankle dorsiflexed to 0° (p<0.05). However, as for the pennation angle of the gastrocnemius, there were no significant changes after each heel-drop exercise. Conclusions: This finding suggest that the heel-drop exercise with ankle dorsiflexed to 0°, 10°, and 20° is effective on the strengthening of the gastrocnemius. Furthermore, the heel-drop exercise with the ankle dorsiflexed to 10° is more effective than with the ankle dorsiflexed to 0°.