A series of polyethylene/CaCO3 composites with CaCO3 content of 50 wt% were prepared using a twin-screw extruder, in which the polyethylene was mainly based on a kind of metallocene linear low density polyethylene (m-LLDPE) and another LLDPE with broad molecular weight distribution (B-LLDPE). The rheological properties of the composites and the morphologies of extrudate’s surfaces were investigated and characterized by capillary rheometer and scanning electron microscopy (SEM). In the extrusion of the composites, there was a critical value of shear rate (or shear stress) where the extrudate’s surface became rough because of the instability of the viscoelastic composites during the flow with shearing.The morphologies of the extrudate’s surface of the composites based on m-LLDPE would get worse as the processing temperature decreased, while would turn much better with the addition of B-LLDPE content even at relatively low temperature.The results demonstrated that the introduction of lower molecular weight fraction from B-LLDPE can both decrease the viscosity of the melts and improve the processability of the composites of high CaCO3 contents, especially at high shear rates.