To reveal the stiffness behavior of a parallel mechanism with considering the effect of the change in geometry due to compliance caused by the external forces, the stiffness characteristics of 3-PRR planar parallel mechanism were studied systemically. The analytical expressions of stiffness of the mechanism were derived on the basis of conservative congruence transformation (CCT) stiffness matrix. Then, the stiffness mapping curves of the 3-PRR mechanism were given to show the change and behavior of the stiffness with and without external forces acted on the mechanism, thus comparing the different changes in the stiffness and how it changes. And the stiffness characteristics of the mechanism were analyzed and discussed in details according to the stiffness expressions and numerical examples. The results show that the stiffness of the 3-PRR mechanism is configuration dependent, and proportional to the actuating forces and joint stiffness. The stiffness correlates not only to the magnitude of external forces but also the direction in which the external force act upon.