PURPOSE: Woodflour is heat treated to remove comstituents of woodflour other than water to produce said wood flour filler showing low volatility and fuming properties when it is filled in thermoplastic resins and formed and giving formed products with high quality.

CONSTITUTION: The heat treatment of woodflour is conducted preferably at about 160W260°C to remove more than 3, preferably 5W15 wt% of constituents of woodflour other than water. During the operation of the heat treatment, the impregnation of the woodflour with a processing aid is desired. The processing aid is, e.g., a medium or higher fatty acid, its metal salt, an ester from a fatty acid and a mono- to octahydric alcohol, a medium or higher fatty acid amide or its derivative, a 12W 26C aliphatic alcohol or an etherified product from an epoxy compound of polyhydric (di- to octahydric) alcohol and the amount is preferably 1W30 wt% based on the dry weight of the woodflour.