In recent decades, stretchable conductive polymers have gained extensive interest of researchers because of their hi-tech applications in electronics, textiles and medicine devices. In this study, carbon nanotubes and carbon nanospheres, as the chemically stable dopants, were uniformly dispersed in a polyurethane matrix to develop a highly elastic and stretchable conductive polymer composite film. The nanocomposite film inherited the advantageous properties from its constituents, namely the high conductivity from carbon nanotubes and nanospheres, and the elastomeric mechanical properties from the polyurethane. The conductive polymer nanocomposite film can be uniaxially and biaxially stretched up to 50% without clearly mechanical or electrical changes. Stretching beyond 50% would result in the conductivity decreasing gradually. Therefore, the as-prepared stretchable conductive polymer nanocomposites possessed both the high conductivity and the high elasticity, which would have greater application potential in high-performance electronic circuits.