The present invention relates to a process for the manufacturing of composite materials from natural fibers and thermoplastic polymers. Examples of fibers are wood fibers originating from pulping processes known as refiner pulp (RMP), thermomechanical pulp (TMP) or chemi-thermomechanical pulp (CTMP), but the process can also be applied to other kinds of natural fiber containing raw materials. In the process according to the present invention, fibers are introduced from the blowline or the housing of a refiner into a flash tube dryer, separated from humid air in a cyclone, introduced into a compounder and mixed with at least one thermoplastic polymer and the product is subsequently pelletized. The process according to the present invention is advantageously run as a continuous process.