Purpose: Children with Down syndrome (DS) have multiple difficulties in gait pattern. So, the effect of the activity level and speed on the gait coordination and variability was investigated. Methods: In this case-control observational study, 24 participants in three groups of active and non-active children with DS, and the control group without intellectual disability were asked to walk on a treadmill with two speeds of 0.8 and 1.2 m/s. Continuous Relative Phase (CRP) and variability of CRP in thigh-leg and leg-foot coupling were assessed. Results: CRP and variability of CRP in the leg-foot coupling in the control group were significantly higher than active and non-active groups with DS. Speed led to increase the CRP of leg-foot in the active group with DS and increase the variability of this coupling in non-active group with DS. Conclusion: In this study, the activity level provided the compatibility with speed changes of walking in CRP of leg-foot in children with DS.