Postsynthetic surface functionalizations of the amphiphilic poly(allyamine)-g-poly(t-butylacrylate) nanocapsule of 50–100 nm diameters are reported. The hydrophobic poly(t-butylacrylate) surface of the polymer nanocapsules were successfully functionalized with poly(acrylate) and poly(ethylene glycol). These new nanocapsules were investigated for their water dispersibility, encapsulation and controlled releasing properties with fluorescein, an anionic fluorescence probe, and 5-fluorouacil, an anticancer drug as the model compounds. The postsynthetic surface funcitionalization approach of the cross-linked polymer nanocapsules shows promise for improving the biocompatibility and aqueous dispersibility of the nanocapules without disrupting the encapsulation and releasing properties of the polymer nanocapsules.