Abstract. This paper focuses on the performance evaluation of parallel robots. The existing evaluation methods have been deeply investigated, and we come to the conclusion that some of the existing performance measures are not suitable and there is a need to introduce new ones. Considering the specialties of parallel robots, we think that the performance with respect to motion/force transmission should be evaluated. Consequently, some indices have been defined based on the reciprocal product of screw theory, and suggested as the evaluation criteria for the optimal design of parallel robots. These indices are frame-free and their values fall into a finite range [0 1]. To illustrate their merits and applications, some cases have also been provided.