Spasticity is a common and disabling symptom associated with Upper Motor Neuron Syndrome. The current methods of treatment for muscle spasticity have been claimed as unsatisfactory. Therapeutic ultrasound (US) is a common therapeutic modality in physiotherapy used for treating a wide variety of disorders. The aim of present study was to quantify the effects of continuous US on ankle plantarflexor spasticity in a randomized, single-blind, placebo-controlled trial. Twelve stroke patients (6 women and 6 men) randomly allocated to ultrasound (n = 6) or placebo (n = 6) groups. The patients were treated with either US or sham US for three days per week, every other day for 15 treatment sessions. The primary efficacy measures were the H-reflex and the Ashworth Scale (AS). In experimental group there was a significant reduction in the Hmax/Mmax ratio (p = 0.03). The Hmax/Mmax ratio was increased in the placebo group patients (p > 0.05). The difference between two groups was statistically significant (p = 0. 02). In both groups there was a reduction in the posttreatment AS scores. The Ashworth scores demonstrated statistically significant changes in the US group (p = 0.04). There was no statistical difference, however, when these two groups were compared (p > 0.05). Results from the present study show that treatment with US can reduce HmaxlMmax ratio as a measure of alpha motoneuron excitability and spasticity measure of AS in stroke patients with ankle plantarflexor spasticity.