Gold nanoparticles have shown great promise in a variety of biological applications including the use in highly sensitive diagnostic assays 1,2, thermal ablation 3, radiotherapy enhancement 4, as well as drug and gene delivery 5. Such gold particles, however, suffer from losing or reducing sensitivity and selectivity due to aggregation under high ion strength of biological fluids and non-specific interaction with biomolecules, such as proteins or DNA. Poly ethylene glycol (PEG), which is known to lengthen the circulation time of biomedicines in the bloodstream, reducing the non-specific binding of proteins, and increasing efficacy and tolerability, is currently used as coating for different kind of nanoparticles to improve their stability and biocompatibility. The currently used strategy was to attach PEG moelcucles to gold nanoparticle through Au-SH chemical bonding. There is no available information, however, about how the length, conformation and attachment sites of PEG moiety affect the binding stability on gold nanoparticles, which play such critical role in retaining the solubility, while facilitating both selectivity and reactivity. In our present work we examined the stability of thiolated PEG with different length and multi-thiol anchors bound to gold nanoparticle by an assay of PEG displacement with different moities including di-thiolthreitol (DTT) and mercaptoethanol. Dynamic light scattering (DLS), Atomic force microscopy (AFM), multiangle-laser light scattering (MALS) incorporated with refractive index (RI), UV and DLS detectors were employed to characterize size, geometry, and packing density of PEG. This information will enable us to understand, optimize and control their efficacy and distribution of gold nanoparticle-based diagnostic or therapeutic agents under complex physiological environments. 1 Tkachenko, A.G.; Xie, H.; Coleman, D.; Glomm, W.; Ryan, J.; Anderson, M.F.; Franzen, S.; Feldheim, D.L. J.Am. Chem. Soc.2003,125,4700.2 Nam, J.M.; Thxton, C.S.; Mirkin, C.A.; Science 2003, 301, 1884.3 Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Nano. Lett. 2005, 5, 709.4 Hainfeld, J.F.; Slatkin, D.N.; Smilowitz, H.M.; Phys. Med. Biol. 2004, 49, N309.5 Thomas, M.; Klibanov, A.; Proc. Natl. Acad. Sci. USA 2003, 100, 9138.