The properties of the composite of polypropylene (PP) and chemithermomechanical pulp reactively treated with bismaleimide-modified PP or premodified pulp have been investigated. The results were compared with the properties of unmodified composites of PP and pulp. The effect of some flame retardants on the properties of composites was also studied. Premodifications of PP as well as pulp with m-phenylene bismaleimide provided a positive response with regard to the mechanical properties of the composites. The tensile strength of a 35 wt % pulp-filled PP composite was found to be much higher than that of the strength of pure PP. On the other hand, tensile strength decreased considerably with increase in the chemithermomechanical pulp content if unmodified PP was used. The occurrence of chemical grafting reactions between PP and bismaleimide as well as between pulp and bismaleimide have been suggested, which can explain the aforementioned difference in mechanical properties among different composites. In situ addition of sodium berate, boric acid, or phenolic resin during processing of the composite decreased the rate of burning of PP.