Objective To evaluate whether therapeutic ultrasound and passive stretching exercises have greater effects on knee extension gain than passive stretching exercises alone. Material and methods Quantitative study of experimental design. A total of 12 mild spastic cerebral palsy patients (between 7 and 12 years) with shortening of the hamstrings. They were randomly divided in 2 groups. A group received triple flexure mobilization, massage, therapeutic ultrasound and a 3 static passive stretching bout and another group received the same intervention without ultrasound. Both, therapeutic ultrasound + passive elongation group (Gr-UST+EP) and passive elongation group (Gr-EP) were evaluated using a goniometer at the angle of the knee at the beginning and end of each session. Then, knee extension gain was measured. The intervention was performed twice a week per 4 weeks. Results The group that received therapeutic ultrasound + passive elongation over 4 weeks of treatment had a higher knee extension gain than the passive elongation group. Both the left knee F (1.9) = 15.386, P=.003 and right knee F (1.9) = 5.690. Conclusions Therapeutic ultrasound and passive stretching exercises has greater knee extension gain than passive stretching exercises alone, so it might be a good therapeutic choice in maintaining joint ranges and avoidance of joint deformity in patients with mild spastic cerebral palsy.