Three sub-10 nm platinum nanoparticles (PtNPs) with distinctive morphologies were developed by using cucurbituril (CB) as stabilizing agent and support. Both the size and shape of the PtNPs were simultaneously controlled by tuning the reducing agents. The prepared NPs have been comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and cyclic voltammetry. On account of the presence of CB and its unique structural features, the as-prepared PtNPs are homogeneous in morphologies and exhibit higher activities toward methanol electrooxidation than commercial Pt/C. CB has the ability to bind small molecules that can promote CO oxidation, therefore, all the three PtNPs showed enhanced poisoning tolerance. Such unique abilities of CB can even promote the poisoning tolerance of commercial Pt/C through simple physical mixing.