To assist the mobility handicapped, it is necessary to support their body weight in part using an assistive device. Since the human leg can be modelled by a spring during the gait, the assistive device (or robot) should have a certain level of compliance in order to achieve natural gait assistance. A pneumatic cylinder is added in parallel to a leg (which is also a spring) for antigravity with compliance. By simulation, the compliant gravity assistance effectively reduces the required forces and torques at the joints of leg. Using this idea, a prototype of an assistive robot named SoftGait is developed and experiments are performed to verify its efficacy.