Background: The terminal sialylation of N-glycans can provide longer half-life of glycoproteins in the blood circulation. Therefore, increasing of sialyation is important in therapeutic glycoprotein production. In this work, we attempted to employ co-expression strategy of CMP-sialic acid transporter (SAT) in Chinese hamster ovary (CHO) cells to obtain improved sialylation of recombinant glycoproteins. CMP-SAT is antiporter and transports cytosolic CMP-sialic acid into the Golgi lumen. Methods: We expressed CMP-SAT in CHO cells producing recombinant human erythropoietin (rhEPO) as a model human glycoprotein. Results: We found that co-expression of CMP-SAT improved sialylation contents (actually, N-acetylneuraminic acid form) of rhEPO. Interestingly, this CMP-SAT co-expression also decreased N-glycoylneuraminic acid contents. Conclusion: This result shows that co-expression of CMP-SAT in CHO cells could enhance sialylation and also its quality.