By introducing the concept of virtual link

the relationship between the crank existence condition of a fourbar chain and the link lengths of the mechanism is analyzed based on the Grash of theorem. The four-bar chain is a virtual equivalent mechanism of planar 3-DOF PRRRRP six-bar mechanism. According to the relationship of the mechanism assembly

the crank existence conditions are obtained under the different relationship among link lengths. A program is developed to determine the crank existence conditions of planar 3-DOF PRR-RRP six-bar mechanism. The results are the design and optimization basis of planar 3-DOF PRR-RRP six-bar mechanism.