Background: Measurement of muscle hardness is important for objective evaluation of the muscle condition. Muscle hardness may be related to muscle size, but this relationship has not yet been examined. Purpose: To examine the relationship between the size and hardness of the medial gastrocnemius muscle (MG) at different ankle joint angles. Material and Methods: Twelve young male subjects comprised the present study. Muscle thickness of the right MG (MT MG) was measured by ultrasonography as an index of muscle size. The MT MG was determined at 30% of the lower leg length from the popliteal crease to the lateral malleolus. At the same site, muscle hardness of the right MG (MH MG) was measured with ultrasound real-time tissue elastography. Both measurements were taken while each subject was instructed to lie in a prone position on a test bench. The right ankle was secured to a handmade apparatus by a non-elastic belt, with the hip and knee joints fully extended and the ankle positioned at +30°, 0° and -20° (0°: neutral position, positive values indicating plantar flexion). Results: There were no significant differences in the MT MG among the ankle joint angles whereas the MH MG significantly increased as the ankle was moved from plantar flexion to dorsiflexion. As a result, there was a difference in the MT MG-MH MG relationship between the angles. The MT MG significantly correlated with the MH MG only at +30° (r = 0.635, P < 0.05) where passive torque around the ankle has been shown to be near zero in previous studies. Conclusion: The hardness of the relaxed MG with no passive tension appears to depend on its size.