Large enhancements of the melt strength of polypropylene (PP) were achieved by the introduction of high molecular weight polyethylene (PE) into PP. The viscoelastic properties of the high-melt-strength PP melts under shear flow were investigated. It was found that the rheological properties of the high-melt-strength PP were distinctly different from those of conventional PP. The elastic response at low frequencies was significantly enhanced in comparison with the conventional PP, implying a presence of a long relaxation time mode that was not revealed in conventional PP, In step-shear measurements, the fast and slow relaxation processes that characterized the linear viscoelastic properties were observed also for nonlinear relaxation moduli. The dependence of the damping for the slow process of the high-melt-strength PP on shear strain was much weaker than that of the fast process. These rheological behaviors characterizing the long relaxation time mode were further enhanced with the increasing concentration of high molecular mass PE. The unusual shear rheological behaviors were discussed in view of the role of high molecular weight PE as a long relaxation time mode within PP.