This paper studies the comparative stiffness analysis of two over-constrained 2-RPU&SPR parallel mechanisms, these two 2-RPU&SPR parallel mechanisms are different in the joint distribution. Firstly, the kinematics of the two mechanisms are analyzed, the inverse and the forward positon problems of the two manipulators are obtained. Then, the dexterity analyses of the two mechanisms are carried out by the derivation of the close-loop positon equation. Thirdly, stiffness analyses of two over constrained structure are carried out by using principle of virtual work, and 6×6 full matrices for the two structures have been built. Finally, some examples and comparisons are enumerated to show the differences of the two mechanisms. © 2016 IEEE.