We reported an effective and environmentally friendly method of decorating Pt nanoflowers (Pt NFs) on the surface of carbon nanotubes (CNTs). By reduction of K2PtCl4 with L-ascorbic acid in an aqueous solution, Pt nanoparticles with a flower-like morphology were deposited onto the surface of poly(sodium 4-styrenesulfonate) wrapped CNTs (PSS-CNTs). PSS plays a dual role: dispersing stable CNTs into solution in water, and providing functional groups that bind metal ions and metal nanoparticles. As-prepared Pt NFs/PSS-CNTs showed a large electroactive specific surface area, and presented excellent electrocatalytic activity for oxygen reduction. Au and Pd nanoparticles could also be deposited on PSS-CNTs. This method could be extended to the synthesis of different kinds of metal nanoparticles on other polymer wrapped CNTs. As-prepared metal nanoparticle/CNT nanocomposites could find use as electrocatalysts in polymer electrolyte membrane fuel cells.