Abstract: The poplar cellulose nanofibers (CNF) were extracted from poplar flour and used to reinforce the wood flour/high density polyethylene(HDPE) composite by physical pretreatment method and polyethylene oxide (PEO) dispersion agent method. Then the wood flour/HDPE/CNF composites were prepared by extruding molding. The effect of dispersion and reinforcement of CNF in the wood flour/HDPE/CNF composites was investigated by a comparioson of two pretreatment methods using compounding method as control. The results indicated that CNF with a diameter of several hundred nanometers could be achieved successfully after the effectively removing of the lignin and hemicellulose of poplar by chemical and mechanical treatments. SEM images showed that an ideal dispersion of CNF in the HDPE matrix of the the wood flour/HDPE/CNF composites could be obtained by using these two pretreatment methods. CNF were entangled with wood flour and HDPE matrix forming a three-dimensional network structure, and the fracture mode of the composites became the ductile fracture. The bending strength and elastic modulus of the wood flour/HDPE/CNF composites prepared with the two pretreatment methods were improved evidently. The bending strengths of the composites by the physical pretreatment method and the PEO dispersion agent method were 43.3 and 38.7Mpa,which was increased by 36.2% and 21.7% respectively compared to the control 31.8 MPa. The elastic modulus of the composites were 3 342 and 3 008 MPa with these two pretreatment methods, increasing by 48.9% and 34.1% than the control 2 243 MPa. The bending strength and elastic modulus of the composites with the physical pretreatment method were higher than those of the composites with the PEO dispersion agent method. This indicated that the physical pretreatment method was an environmental friendly and effective method.