The present disclosure relates to an antibody-drug conjugate in which a modified antibody comprising a motif having a specific structure at the end of the antibody is conjugated to a drug via a linker, and a composition comprising the same, and more particularly to a modified antibody-drug conjugate (mADC) comprising a modified antibody that has a significantly increased conjugation yield of drug due to a motif bound to the heavy chain or light chain C-terminus of the antibody, and to a composition comprising the same.