To meet the demanding in modular design of robotized devices for welding, cutting, deburring and polishing, etc, a novel 6-DOF hybrid robot compose of a plane symmetrical 3-UPU parallel mechanism plus a 3-DOF rotating head is presented. With a brief introduction to the architecture of the robot, a semi-analytical stiffness model of the 3-UPU parallel mechanism is formulated by analyzing the wrenches systems imposed by the UPU limbs upon the platform. This leads to the complete formulation of the static and deformation equations, resulting in a comprehensive stiffness model that accounts for the compliances of all limb components and joints. With the aid of this approach, the stiffness of a prototype machine throughout the entire task workspace is evaluated. The computational results are validated by the ANSYS at two typical configurations.