In this papec the dimensional design methodology for a novel planar three degrees of freedom parallel machine tool is investigated. Closed-form solutions are developed o r both the inverse and direct kinematics. Based on the formulation ofthe local dexterity and the dejinition of the theory workspace and valid workspace, the effects of the design parameters on the dexterity and workspace were discussed. The change ratios of' condition number and valid workspace were defined anti used as the evaluated indicator to analyze the effects oj' the design parameters on the performances. According to the analysis results, an approach that could satisfy the requirement of motion platform realizing orientation cupability, dexterity and valid workspace was proposed. An design example is given to illustrate the effectiveness of this approach.