The effects of various flame retardant additives on thermal degradation of polyurethane, which was synthesized from environmentally-friendly modified castor oil, were investigated through thermogravimetric analysis and limit oxygen index analysis. The deterioration in physical and physicomechanical properties of these materials due to additives usage were also monitored in order to ensure optimal composite results. Irregularities in plastic matrix caused by bad additive dispersion were observed based on analyzing scanning electron microscope images. Based on those results, a combination of flame retardant additives was determined, which not only significantly increase thermal stability of polyurethane, but also reduces toxic smoke emissions through favouring char formation in thermal degradation process of the plastic matrix.