This investigation developed a measure of motor control at the ankle for persons with CP using relative phase. Twenty-nine subjects, 14 with spastic diplegia cerebral palsy (CP group) and 15 without disability (WD group) were tested once. Video data were collected as a seated subject performed four full range of ankle plantar and dorsiflexion movement tasks (right ankle, left ankle, ankles in-phase with each other, and ankles antiphase to each other) at four different frequencies (self-paced, 0.5, 0.75, 1.0 Hz). The relative phase measure was able to discern the differences between the two groups of children. The CP group had poorer motor control than the WD group, based upon the measure. Both groups had more difficulty performing the antiphase than the in-phase movements. The investigation adds to the body of knowledge in that the concept of relative phase was used as a measure of motor control at the ankle in persons with CP. Results indicated that the measure was adequately sensitive to quantify differences between a group with CP and a group without disability. Clinically the measure could eventually be used as both an assessment and outcome tool.