A rotor indexing system is provided for indexing the massive high inertia hammer-carrying rotor (5) of a hammer mill (1) or the like. The rotor is mounted on a large main shaft (2) which is rotatably driven by a main drive motor (4). A motively powered indexing apparatus is disposed at and in axial alignment with the rear end of the main shaft, and includes a power transmission or coupling (12) comprising a first coupling member (14,56) coaxially mounted to and for rotation with a rotatable output member (the rear end of the main shaft in this embodiment) and fixed against longitudinal axial movement; as well as a second coupling member (15,58) normally spaced from the first coupling member and mounted for longitudinal movement into and out of full coupling power-transmitting engagement with the first coupling member. The second coupling member is mounted to a prime mover or rotor indexing drive assembly (7) including a motive means (9b) of relatively high speed and low torque connected through a speed reducer (10) having an extremely low speed high torque output shaft (11), to which the second coupling member is connected. A normally engaged brake (9a) is connected to the assembly motor input shaft, and is arranged to disengage when the assembly motor is actuated. The arrangement may be such that the entire rotor indexing drive assembly is movable and carries the second coupling member into or out of selective engagement with the first coupling member.