OBJECTIVE: This study was designed to evaluate the effect of stretching on decreasing spasticity of ankle plantar flexor muscles by biomechanical assessments. METHOD: Twenty two ankle joints of nineteen patients with upper motor neuron lesion were included. The spasticity was assessed both clinically and biomechanically before and after stretching of ankle plantar flexor muscles by tilt table. For clinical assessment modified Ashworth scale (MAS) was used. For biomechanical assessment, ankle plantar flexor muscles were stretched isokinetically while EMG signals were recorded simultaneously and peak eccentric torque, stiffness index and stretch reflex threthold speed (SRTS) were measured. RESULTS: Two cases showed improvement in MAS after stretching but the others did not. SRTS of ankle plantar flexor was increased significantly while peak eccentric torque and stiffness index were unchanged. CONCLUSION: Passive stretching of ankle plantar flexor muscles decreased the stretch threshold, that is a neural component of spasticity but it did not decrease the mechanical component of spasticity.