Wood-plastic composite (WPC) has been made by impregnating methylmethacrylate (MMA) monomer into simul, a native timber of Bangladesh, under Co-60 γ radiation. The effect of moisture and heat and humidity on simul and its WPC is investigated as a simulating phenomenon of the sunlight, rain, and moisture to which timber is generally exposed. Mechanical properties measured under variable conditions of moisture, heat, and humidity reveal that the WPC is a much improved product, suitable for use as a substitute for the good-quality timber normally applied for household goods.