In this study, rheological behaviors and capillary extrusion flow instabilities of ultra-high molecular weight polyethylene (UHMWPE)/high-density polyethylene (HDPE)/SiO2 composites containing modified nano-SiO2 or pure nano-SiO2 are investigated. Effects of interfacial conditions between the dispersed nano-SiO2 phase and PE matrix on rheological behaviors are analyzed. The results show that modified nano-SiO2 in the PE matrix has a relatively strong interfacial interaction with the polymer chains compared with pure nano-SiO2, thus causing a more pronounced shear thinning behavior and reducing extrudate swell during capillary extrusion. Nanocomposite extrudates experience the transition of smooth- sharkskin- oscillating distortionoverall melt fracture with the increase of shear rate (or shear stress). The interfacial interaction allows a more storage of elastic energy as the melt flow through the die, resulting in sharkskin distortion and oscillating distortion to occur prematurely at critical shear rates. It also restricts the elastic recovery of the melt after leaving the die, thus delaying sharkskin distortion under shear stress. At high shear rates, the modified nano-SiO2 particles begin to roll, and some of the unmodified particles move and embed to the wall. It is believed that the interfacial adsorption effect and the wall-slip effect of nanoparticles exist simultaneously in the whole extrusion process.