The melting and crystallization behaviors, morphology, and mechanical properties of polypropylene (PP)/surface-treated calcium sulfate (CaSO4) whisker (T-CSW), β-PP/T-CSW, and β-PP/polypropylene- graft-maleic anhydride (PP-g-MAH)/T-CSW composites had been investigated via differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), polarized light microscopy (PLM), scanning electron microscopy (SEM), and mechanical tests. We found that T-CSW was an α-nucleating agent and increased the crystallization temperatures of PP, but PP-g-MAH and high loadings of T-CSW had weakly negative effects on the crystallization rates of PP. The T-CSW restrained the formation of β-spherulites, and the spherulitic size decreased in the composites. PP-g-MAH improved the compatibility and adhesion between T-CSW and the matrix. The notched impact strength was improved, and the tensile strength was enhanced at low levels of T-CSW, while the flexural modulus was weakened for β-PP/T-CSW and β-PP/PP-g-MAH/T-CSW composites versus PP/T-CSW composites. POLYM. COMPOS., 37:2121-2132, 2016. © 2015 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]