Glycosylation is an important post-translational modification of protein. The types of glycosylation of protein can be classified into N-glycosylation and O-glycosylation, depending on the types of connection between the peptide and sugar. N-glycosylation is connected by the N-acetyglucosamine (Glc-NAc) at the reduced end of the sugar chain and the nitrogen atoms on some of the Asn side chain amino groups in the peptide chain. The Asn connected to the sugar chain must be in the motif made up of Asn-X-ser/Thr 3 residues, in which X is an arbitrary amino acid residue with the exception of Pro. The structure of O-glycosylation is simpler than N-glycosylation, and the sugar chain is short, but with much more diversities. O-glycosylated peptides are mainly linked to sugar via Ser and Thr residues, however, some types of O-glycosylation may be connected by tyrosine, hydroxyllysine and hydroxyproline residues.