Abstract: Three types of highly filled polypropylene (PP) nanocomposites were investigated by a capillary rheometer, and the effects of die temperature, shear rate, melt pressure, nano-particle filling ratio and morphology of nanoparticle on the melt-extrusion swell ratio of PP nanocomposites were studied. The results indicated that these three types of PP nanocomposites presented a decrease in melt extrusion swell ratio with an increase of die temperature by a linear relationship. The melt extrusion swell ratio also increased with an improvement of shear rate but decreased with an increase of filler filling ratio. However, it increased with an increase in melt pressure and melt pressure. There was a retarded decreasing trend observed in melt extrusion swell ratio with an increase of filling ratio. The flake nanoMg (OH)2 filling system exhibited the smallest melt extrusion swell ratio in the three composite systems at the same volume fraction and temperature, but the spherical nano-CaCO3 filling one presented the greatest value.