We have investigated the effects of B and C on the ordering of L10-CoPt films and confirmed the following quite different role between B and C in CoPt. A small amount of B considerably decreases the temperature for ordering and is interstitially incorporated into L10-CoPt along its c axis due to the large negative heat of solutions with Co and Pt. X-ray photoelectron spectroscopy analyses have revealed that B atoms combine with Co and Pt. In contrast, C is not incorporated into the CoPt lattice due to its positive heat of solution, resulting in no decrease in the ordering temperature. The present results reveal that CoPt films containing a small amount of B are promising for high density recording media.