Multi-walled carbon nanotubes (MWNTs) have been noncovalently modified with poly [2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly [vinyl pyrrolidone] (PVP) to form two soluble MWNTs/polymer supramolecular composites. For the composite MWNTs/MEH-PPV, there is π-π interaction between the MEH-PPV and MWNTs besides wrapping of the polymer. The ultrafast third-order optical nonlinearities of the two composites were measured by using femtosecond time-resolved optical Kerr technique at the wavelength of 820 nm. MWNTs/MEH-PPV exhibits much larger ultrafast third-order optical nonlinearity than MWNTs/PVP. This enhancement is ascribed to the strong intermolecular nonlinearity induced by the π-π interaction between the polymer MEH-PPV and the sidewall of carbon nanotubes. © 2005 Elsevier B.V. All rights reserved.