Starch was treated with three kinds of compatibilizers (coupling agents or modifying agents), KH-550, KH-570, and glycerin monostearate. Blends of polypropylene (PP) and treated starch were prepared by a twin-screw extruder. The effects of the starch before and after treatments and the kinds and contents of the compatibilizers on the mechanical and thermal properties of the PP/ starch blends were investigated in this study. We found that the mechanical properties (tensile strength, impact strength, and elongation at break) of the blends were obviously improved with increasing content of different kinds of compatibilizers. Meanwhile, the most significant improvement in the mechanical properties was obtained in the samples containing just a 1 wt % loading of compatibilizers, and KH-570 had the best improved effects among the different kinds of compatibilizers. The results of thermogravimetric analysis demonstrate that to some extent, the thermal stability of the PP/starch blends was improved after the addition of compatibilizers. Scanning electron microscopy showed that the dispersion of starch in the PP matrix and adhesion between the starch and PP matrix were obviously improved after the addition of compatibilizers. KH-570 not only had the best improved effects among the coupling agents but also still acted as a similar plasticizer.