Effects of high shear induced melt-mixing process on the flexural properties of PP/ montmorillonite (MMT) composites were investigated. For discussion of that effect, filler dispersion was also observed by using a field emission type scanning electron microscope (FE-SEM). The results showed that flexural properties were improved by the improvement of MMT dispersion due to high shear induced melt-mixing process. In addition, the effect depended on the mixing speed and the time. Longer mixing time and Faster mixing speed resulted in the reduction of flexural strength and the improvement of the modulus. The exfoliation of MMT platelet was progressed by longer mixing time and faster mixing speed, which means an increase of mixing energy. On the other hand, degradation of polymer was also progressed, resulting in the reduction of flexural strength and the improvement of the modulus.