Pegylation of gold nanoshells provides an effective means to reduce their reticuloendothelial system (RES) clearance in body. In this study, we perform a parametric investigation on the factors that would affect the macrophage uptake of gold nanoshells with the aim to optimize their pegylation and minimize their macrophage uptake. We synthesized and pegylated the gold nanoshells using methoxy-poly(ethylene glycol)-thiol and employed an in vitro macrophage assay to examine the effect of surface density of poly(ethylene glycol) (PEG), chain length of the PEG, and size of the gold nanoshells on their macrophage uptake. We have shown that a saturated surface density would minimize macrophage uptake, which could be obtained by experimental titration-based Ellman's reagent. Our results suggest that the chain length of PEG and size of gold nanoshells influence the surface density of PEG. We have also shown that PEG with molecular weight of around 2000Da and a size range larger than 186nm would be appropriate for facilitating a high surface density. Our in vitro macrophage system thus provides a good model to accurately predict the RES response to different pegylation parameters.