Wood-Plastic Composite(WPC)with the woodflour contents from 26.5% to 55.8% was prepared using an industrial scale extruder, and the decay resistance was evaluated by fungus cellar test and soil buried test. A mass loss exceeding 5% was obtained in specimens placed in a fungus cellar for four years in specimens having woodflour contents exceeding 40%. Specimens buried in soil for three years at Tsukuba test site showed increased mass loss with increasing woodflour content. Test specimens having 40% or more of the woodflour content showed a mass loss of 12-20%, however, mass loss for specimens having 30% or less wood content was negligible.
The moisture infiltrates into WPC gradually in a long-term exposure, and it is a possible for the moisture content to reach level where decay could occur near the surface at the WPC. Special attention should be paid for WPC having 40% or more of the woodflour content, if they are placed in a severe condition like ground contact that can reach the moisture content sufficient for decay.

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