A roller screw includes a long shaft, a screw nut sleeved onto the long shaft, and rollers, a turnaround member and a turnover member set in between the long shaft and the screw nut. The turnaround member enables the rollers to circulate. The turnover member defines a turnover passage extending over the peripheral surface of the long shaft and connected between two load paths in the long shaft and the screw nut to constitute a circulation path. The direction of arrangement of the rollers before entering the turnover passage is different from that after moving out of the turnover passage so that the amount ratio of the rollers that are arranged in two different directions is kept constant for industrial application.