Biodegradability and interesting physical and mechanical properties of polypropylene (PP)/cellulose composites are the essential motive for undertaking fundamental studies of various properties of such materials. An addition of lignocellulosic particles to polymers results in a change in properties of the product obtained which is a resultant of properties of lignocellulosic material and those of polymer and depends on the proportion of both materials. It is also reflected in the flammability characteristics of the composites. In this study we have analyzed flammability of PP composites with long flax fibers obtained by compressing. The amounts of natural fibers were 12.5%, 20%, 30% and 40%. The thickness of the investigated samples was 5 mm. The samples were tested in accordance with ISO 5660 (Cone Calorimeter) at heat flux of 35 kW/m(2) Heat release rate (HRR) and mass loss rate (MLR) curves show that thermal decomposition and combustion of the mentioned samples occur in a different way. When flax fiber content was 12.5%, a reduction in maximum heat released (HRR peak) by about 35% was observed. However. time to sustained ignition of composites was shorter than that of PP. The characteristics of the composite are closer to those of lignocellulosic material. The addition of 12.5% fibers resulted in an increase in fire retardancy of the composite compared to polypropylene alone. The addition of fibers in the amount exceeding 20% made the composite material characteristics similar to those of lignocellulosic materials.