Wollastonite reinforced polypropylene (PP/CaSiO3) composites were prepared by melt extrusion. A silane coupling agent and a maleic anhydride grafted PP (PP-g-MA) were used to increase the interfacial adhesion between the filler and the matrix. The increased adhesion observed by scanning electron microscopy (SEM) resulted in improved mechanical properties. A model was applied to describe the relationship between the interfacial adhesion and tensile properties of PP/CaSiO3 composites. There is stronger interfacial adhesion between silane-treated CaSiO3 and polymer matrix containing PP-g-MA as a modifier. Results of dynamic mechanical thermal analysis (DMTA) showed that stronger interfacial adhesion led to higher storage modulus. The influence of CaSiO3 particles on the crystallization of PP was studied by using differential scanning calorimetry (DSC). The introduction of CaSiO3 particles does not affect the crystallization temperature and crystallinity of PP matrix significantly.