The loss of muscular strength and balancing disability make it difficult for stroke patients to move their body against gravity or to maintain balance. The asymmetrical kinematics and dynamics between the healthy and affected sides of stroke patients increase the risks of falls. This paper presents the development of a robotic-assisted Sit-to-Stand (RA-STS) mechanism to help stroke patients complete the STS movement essential for subsequent training and restore muscle functions. To provide a rational basis for the design/control of a RA-STS system, a relatively complete set of analytical models is presented for analyzing the asymmetric effects on the human joints during STS. Experiments were conducted to validate the concept feasibility and the kinematic and dynamic models by comparing simulations with experimentally measured results.