Oligo(ethylene glycol) methacrylate (OEGMA) was grafted from Au surface via surface-initiated atom-transfer radical polymerization (s-ATRP). The initiator density was adjusted by the mole fractions of surface-attachable ATRP initiator and diluting agent. Three OEGMA monomers of Mr 300, 475, and 1100 were used. The Au surfaces before and after modification were characterized by water contact angle and ellipsometry. The effect of monomer Mr and initiator density on surface chain density and EO unit density was investigated. It was found that at low initiator density (mole fraction of initiator of 2%–5%), poly(OEGMA) grafts had comparable surface chain density independent of OEGMA Mr. However, at high initiator density (mole fraction of initiator >10%), poly(OEGMA)1100 had obviously lower chain densities than poly(OEGMA)300 and poly(OEGMA)475, which could be attributed to the size difference of the monomers. The short chain length and low surface chain density of poly(OEGMA)1100 resulted in its relatively low surface EO unit density.