Glycosylation is one of the most important post-translational modifications in mammalian cells. This catalytic reaction is processed by adding the various functional sugar moieties to protein through ER and Golgi apparatus. Most therapeutic proteins for human diseases are glycosylated and the glycan structures have been shown to affect therapeutic efficacy. In vivo half-life of therapeutic glycoproteins, in particular, is determined mainly by sialic acid capping at the end sites of N-linked glycans. The non-sialylated glycoproteins have significantly shorter in vivo half-life, because the exposed galactose residues are recognized, captured by asialoglycoprotein receptors (ASGPR) in hepatocytes and immediately degraded. Thus, it is greatly important to increase the terminal sialic acid contents of therapeutic glycoproteins for longer acting of therapeutics. In this presentation, the enhancement of sialylation of recombinant human erythropoietin as a model protein in CHO cells through sialylation pathway engineering approach will be discussed with the experimental data.