We have investigated the effect of the length of multiwalled carbon nanotubes (MWNT) on the dewetting dynamics of thin polymer films. The results indicated that long nanotubes were much more effective than short nanotubes in stabilizing the films against dewetting. The diffusion of polymer chains in the filled matrices was measured using neutron reflectivity, and the result indicated no significant effect on the diffusion coefficient by either short or long nanotubes. We therefore proposed a model whereby the nanotubes did not interact with the individual polymer chains. On the other hand, the long nanotubes formed an effective entangled network, which prevented long-range motion of the polymer films upon dewetting. This model was supported by rheological experiments on bulk samples where the nanotubes had a strong effect on G' of the polymer and only a negligible effect on G ''.