Two kinds of compatilizers, maleic anhydride grafted polyolefin elastomer (POE-g-MAH) and maleic anhydride grafted polypropylene (PP-g-MAH), were incorporated into a polypropylene/magnesium oxysulfate whisker (PP/MOSw) composite. Scanning electron microscopy pictures presented a clear interface between MOSw and the PP matrix in the PP/MOSw composite, while vague interfaces appeared in the PP/iPOE-g-MAH/5MOSw and PP/iPP-g-MAH/5MOSw composites. Dynamic mechanical thermal analysis results indicated that PP-g-MAH was highly compatible with the PP matrix while POE-g-MAH was not. Impact strength results showed that POE-g-MAH had a superior toughening effect on PP/MOSw composites, since the proper interfacial interaction and appearance of beta-crystal PP. However, incorporating PP-g-MAH seemed to be conducive to increasing strength and modulus (both for tensile and flexural tests), as evidenced by the greatly raised interfacial adhesion between the PP matrix and MOSw. Quantitative characterization carried out by Turcsanyi equation for ternary composites also confirmed that PP-g-MAH efficiently enhanced interfacial interaction, by the proof of higher B values. Therefore, the thermal stability of PP-g-MAH treated composites was far superior to that of PP/iPOE-g-MAH/5MOSw composites. Differential scanning calorimetery and polarized light microscopy results showed that POE-g-MAH promoted PP nucleation, with effects further enhanced with the presence of PP-g-MAH.