This paper analyzes the stiffness characteristics of a proposed tripod leg mechanism which is built as based on a 3-UPU parallel manipulator. The description of the mechanical design and kinematic analysis of the leg mechanism are briefly presented. By considering the compliances subject to actuators, the stiffness matrix is derived intuitively as based on the Jacobian matrix of the manipulator. The stiffness performance is evaluated by utilizing the extreme stiffness values over the usable work space, which will be helpful for the gait design of a biped locomotor. Furthermore, the influences of architectural parameters on stiffness properties are presented, which will be useful for the mechanical design of the tripod leg mechanism.