Polymer composites with polyethylene (PE), microcrystalline cellulose (MCC), and coupling agent (PE-g-MA) were melt blended using a single screw extruder (SSE), twin screw extruder (TSE), and a combination of SSE and extensional flow mixer (EFM) and masterbatch method (MB). The rheological behavior, mechanical, thermal, and morphological properties were investigated to compare material behavior of the MCC-filled PE composites and to evaluate the dispersive mixing efficiency. The composite samples prepared using a single pass method in SSE had relatively larger storage moduli (G') while those from the TSE and SSE + EFM were smaller. The complex viscosities (eta*) of composites with processed using EFM was higher than that of neat PE, indicating better dispersion of the MCC. MCC-filled PE composites processed with EFM showed higher tan delta compared to those of composites. In general, MCC-filled PE composites from the EFM process showed higher viscosity compared to all other composites at all shear rates. The EFM method improved flexural properties more than the other processing methods. The effect of EFM on impact strength of the composites was improved with coupling agent. Addition of MCC, couple agent, and different mixing methods did not significantly affect the thermal properties of the MCC-filled PE composites.