Abstract: The theories of error analysis, optimization and virtual displacement are combined together to discuss the pose error of planar mechanism with joint clearance, and a calculation method is proposed for the pose error of planar mechanism. Based on the principle of virtual displacement, the mapping relation between the original error of a planar mechanism and its output end pose error is analyzed. Then, a mathematical model of the original error that is commonly found in planar mechanism is presented, and the pose error for the output end is also derived. Finally, an extremum expression is explored by virtue of the idea about optimization for the output end pose error. In comparison with other similar methods, the proposed one has the advantages of clean reasoning and high versatility without specifically external force required to act on.