Shear deformation and fracture behaviors in polypropylene (PP)-based model thermoplastic polyolefins (TPOs) were investigated with the Iosipescu shear test. The shear deformation process was monitored in situ via video camera to obtain experimental shear stress-strain curves of model TPOs. Shear fracture mechanisms were studied with optical microscopy and scanning electron microscopy. Macroscopically, the cracks in neat PP propagated along the maximum shear plane, which indicated that mode-II shear failure existed in neat PP. Microscopically, it was shown that shear fracture initiated in the form of partial, discontinuous inclined microcracks that later coalesced and formed the final continuous crack. The incorporation of rubber in PP could transform the shear fracture process into a stretching process in the shear damage zone.