A method of cleaning off organic deposition material accumulated on a mask includes forming an organic deposition material pattern on a substrate using the mask, which includes a plurality of slots, in a deposition chamber including a deposition source; transporting the mask to a stock chamber that is maintained at a vacuum and adjacent to the deposition chamber; and partially cleaning off the organic deposition material accumulated along the boundaries of the slots of the mask in the stock chamber. A system to clean off an organic deposition material accumulated on a mask having a plurality of slots, includes a deposition chamber including a deposition source; and a stock chamber that is maintained at substantially the same vacuum as the deposition chamber and includes a cleaning device that cleans off the organic deposition material accumulated on the mask.