Motion/force transmissibility is an essential property reflecting the kinematic performance of parallel robots. Research on this performance of the single-platform parallel robots (SPPRs) has long been concerned and studied. In contrast, although many innovations and applications of the high-speed articulated-platform parallel robots (APPRs) have been presented, few studies on their motion/force transmissibility have been reported. This paper deals with the motion/force transmissibility analysis of high-speed parallel robots with articulated platforms. A modified output transmission index (MOTI) for the high-speed parallel robots with articulated platforms is proposed based on a newly defined concept of equivalent transmission wrench screw. Furthermore, by having an insight into the instantaneous relative motion inside the mobile platform, a medial transmission index (MTI) is proposed to evaluate its internal motion/force transmissibility. Based on these foundations, the local transmission index (LTI) is redefined as the minimum value of the input, modified output, and medial transmission indices. Under the framework of the above performance indices, motion/force transmissibility analysis of two typical high-speed articulated-platform parallel robots, i.e., Heli4 and Par4, are presented. The proposed indices are excepted to be applied to the optimal design of high-speed parallel robots with articulated platforms.