Waste wood particles of different shapes were used to reinforce high density polyethylene (HDPE) to prepare high performance wood/plastic composites (WPC). The wood particles showed a needle-, flake-, strand-, and powder-shape, respectively. The mechanical and water-uptake properties of the composites manufactured using extrusion were tested. The wood needles/HDPE composites (WN-WPC) exhibited slightly higher mechanical strengths and lower density than the composites with the other shapes of wood particles. When the flake-, strand-, and powder-shaped wood particles were replaced by wood needles, the flexural, tensile and impact strengths were increased approximately by 4.24%, 7.61%, and 16.6% on average, respectively, and the density was decreased by about 7.34%. The length and width of WPC specimens changed little when soaked in water, with the thickening on the edge area only. WN-WPC performed the best waterproof stability. Based on the experimental results, the wood needles with lager length-to-diameter ratio would be the most suitable for producing high performance WPC.