Flax and hemp shives were ground to grain size below 1.6 mm, optionally impregnated with fire retarding agent (product of condn. of silicates with urea polyphosphates and B compds.), modified by acetylation, dried and used for reinforcing polypropylene matrix by coextrusion. The composites were studied for the flammability parameters. Addn. of the shieves (5 to 30%) resulted in a decrease of the total heat release, heat release rate, efficient combustion heat and mass loss rate.