In this study, the effect of heparin-derived oligosaccharide on bovine vascular smooth muscle cell (VSMC) proliferation and signal transduction mechanism was investigated. Extracellular-signal-regulated kinase (ERK) 1/2 has been implicated in the regulation of various cellular functions including proliferation, and we sought to define a functional role for ERK 1/2 in an established proliferation model in order to find a possible mechanism for inhibition of VSMC proliferation by heparin-derived oligosaccharide. The VSMC proliferation model was developed by platelet-derived growth factor (PDGF), and the level of ERK 1/2 protein and messenger RNA was determined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunocytochemical methods. Flow cytometry analysis indicated that heparin-derived oligosaccharide blocked PDGF-induced cell cycle progression by arresting cells in the G0/G1 phase. The results imply that heparin-derived oligosaccharide inhibits VSMC proliferation by moderating the gene and the phosphorylation levels of ERK 1/2, eventually blocking G1/S transition, may be one of the mechanisms for inhibition of VSMC proliferation by heparin-derived oligosaccharide.