A substrate centering device for an organic material deposition system comprises: a plurality of substrate support holders configured to be reciprocally movable in a facing direction within an organic material deposition chamber and supporting both side portions of a substrate loaded by a robot; a substrate centering unit configured to be reciprocally movable at each of the substrate support holders and centering the substrate by guiding both side portions of the substrate; and a plurality of substrate clampers configured to be reciprocally movable in a vertical direction at each of the substrate support holders, and clamping the substrate that has been centered by the substrate centering unit.