Mucins are heavily O-glycosylated glycoproteins found in mucous secretions and as transmembrane glycoproteins of the cell surface with the glycan exposed to the external environment. In mucins, O-glycans are covalently α-linked via an N-acetylgalactosamine(GalNAc) moiety to serine or threonine, and the structures are named mucin-type O-glycans. Mucin-type O-glycans are initiated by UDP-GalNAc∶ polypeptide N-acetylgalactosaminyltransferases, which enzymatic mechanism and structural features have been a hot topic of glycosyltransferases research. Mucin-type O-glycans of cancer cells are often changed, both in structure and in quantity, developing several cancer-associated glycans, such as T and Tn antigens. These structural changes can alter the function of the cancer cells, and its antigenic and adhesive properties, as well as its potential to invade and metastasize. These cancer-associated glycans can be exploited to tumor diagnosis, and in the development of anti-tumor drug or vaccine.