Abstract: The analytical stiffness equations of the part concerning joint stiffness and configuration and the uncompleted analytical stiffness equations of the part affected by external forces of 3-RRR planar parallel mechanism were all deduced, based on the conservative congruence transformation (CCT) stiffness matrix. The method of stiffness mappings was given, and the maps of the mechanism were presented to illustrate the properties of the stiffness of the mechanism. The stiffness characteristics of the mechanism were analyzed and discussed. It was revealed that the stiffness of the mechanism is configuration dependent and proportional to the actuating forces and joint stiffness. The stiffness becomes higher towards the boundaries of working space and vise versa, which forms a stronger stiffness 'ring' area near the boundaries, and a weaker stiffness 'zone' near the center of working space.