The invention relates to a process for manufacturing composite material which contains fiber based and polymer based material, wherein the manufacture includes a preliminary treatment process to form a raw stock mixture. According to the invention, the preliminary treatment process includes a hot-cold mixing device which comprises at least a heating stage, and that raw stock, which contains lignin-free material, and a coupling agent are fed to the mixing device to form the raw stock mixture, and adhesion between fiber based and polymer based substances of the raw stock is ameliorated by the coupling agent.