The shear rheological properties of polystyrene (PS)/nano-CaCO3 composites were studied to determine the plasticization of nano-CaCO3 to PS. The composites were prepared by melt extrusion. A poly(styrene-butadiene-styrene) triblock copolymer (SBS), a poly(styreneisoprene-styrene) triblock copolymer (SIS), SBS-grafted maleic anhydride (SBS-MAH), and SIS-grafted maleic anhydride were used as modifiers or compatibilizers. Because of the weak interaction between CaCO3 and the PS matrix, the composites with 1 and 3 phr CaCO3 loadings exhibited apparently higher melt shear rates under the same shear stress with respect to the matrix polymer. The storage moduli for the composites increased with low CaCO3 concentrations. The results showed that CaCO3 had some effects on the compatibility of PS/SBS (or SBS-MAH)/CaCO3 composites, in which SBS could effectively retard the movement of PS chain segments. The improvement of compatibility, due to the chemical interaction between CaCO3 and the grafted maleic anhydride, had obvious effects on the rheological behavior of the composites, the melt shear rate of the composites decreased greatly, and the results showed that nano-CaCO3 could plasticize the PS matrix to some extent. Rheological methods provided an indirect but useful characterization of the composite structure.