The characteristics of monoclonal antibodies (mAbs) cohering various function effectors shows great expectation in therapy. Glycosylation, one of the common post-translational modifications, deeply influences the cohesion. It is necessary to grasp monosaccharide composition/sequence and glycan structures in mAbs. There have been comprehensive mass spectrometry characterization of N-glycosylation of mAbs, and monosaccharide compositions are deduced according to known biosynthetic rules. Our recently developed intact N-glycopeptide search engine GPSeeker has made unambiuous structure-specific characterization of N-glycosylation possible with structure-diagnostic fragment ions from selective fragmentattion of N-glycan moieties.