The synergistic effects of multi-walled carbon nanotubes (MWNT) and graphite on the structural features and mechanical and electrical properties of polyurethane (PU) nanocomposites were investigated as functions of filler content. SEM images of the PU/hybrid nanocomposite exhibit that the MWNTs and graphite are dispersed well in the PU matrix and form an interconnected network structure. Accordingly, the tensile strength and strain-at-break of the PU/hybrid nanocomposites were much higher than those of the PU/MWNT and PU/graphite nanocomposites at the same filler content. In addition, it is found that the PU/hybrid nanocomposite containing 1.25 wt% MWNTs and 1.25 wt% graphite shows the electrical conductivity of $1.2{ imes}10^{-4}S/cm$, which is higher by three orders than that of PU/graphite nanocomposite containing 20.0 wt% graphite. The highly improved mechanical and electrical properties of the PU/hybrid nanocomposite are thought to be due to the interconnected network structure of MWNTs and graphite in the PU matrix.