New nanocomposite films were prepared with atactic polypropylene as the matrix and either of three types of cellulose whiskers, with various surface and dispersion characteristics, as the reinforcing phase: aggregated without surface modification, aggregated and grafted with maleated polypropylene or individualized and finely dispersed with a surfactant. Films obtained by solvent casting from toluene were investigated by means of scanning electron microscopy, dynamic mechanical analysis, and tensile testing. In the linear region, the mechanical properties above the glass-rubber transition were found to be drastically enhanced for the nanocomposites as compared to the neat polypropylene matrix. These effects were ascribed to the formation of a rigid network with filler/filler interactions. In addition, interactions between the filler and the matrix as well as the dispersion quality were found to play a major role on the mechanical properties of the composites when investigation of the films was performed in the nonlinear region.