Nanosize CaSO2 and Ca-3(PO4)(2) fillers were synthesized with an in situ deposition technique, and their sizes were confirmed by X-ray diffraction. CaSO4 was prepared in 12- and 22-nm sizes, and Ca-3(PO4)(2) was prepared in 13- and 24-nm sizes. Experimental variables, such as torque, shear viscosity, shear stress, and shear rate, of the nanofilled polypropylene (PP) composites were measured with torque rheometry and melt flow index (MFI) measurements. Torque versus time, shear viscosity versus weight percentage, and MFI versus weight percentage were plotted to investigate the rheological behavior of the nanofilled composites. The Cross-Williamson (CW) model was simulated with the MATLAB simulation package to Study the thirming behavior of the PP composites. The experimental results show a decrease in the shear viscosity with increasing weight percentage of filler. Shear thinning in the molten PP composites was comparatively greater with decreasing nanosize of CaSO4 and Ca-3(PO4)(2). This kind of behavior was confirmed by the N parameter as determined from the CW model. The simulation of experimental data also showed similar trends as the theoretical data. At a certain stage, a violation of theoretical data was observed. This was because of practical limitations of the equation, as the equation does not include consideration of the physical situation of chain entanglements. (c) 2006 Wiley Periodicals, Inc.