A novel mixer based on elongational flow was used to prepare HDPE/MMT nanocomposites with different MMT contents. The effect of MMT and elongational stress in the performance of HDPE/MMT nanocomposites was studied. X-ray diffractometery (XRD) was used to investigate the intercalation effect and transmission electron microscopy (TEM) was used to observe the dispersion of MMT interlayers in matrixes. The results showed that an intercalated structure would be acquired on the nanocomposites; and exfoliated systems were also obtained. From the differential scanning calorimetry (DSC) and thermogravimetric (TGA) curves it could be clearly observed the addition of MMT can improve the thermal properties of the nanocomposites. Mechanical test showed the tensile strength and elongation at break increased at first and then decreased, with the increasing of MMT loading level.