Thermoplastic compositions containing a natural filler and, more specifically, thermoplastic compositions containing a natural filler and having improved thermal and weathering resistance. The wood-plastic composites utilize a cellulose filler, such as wood flour, and vegetal fibers, such as Curauá fiber, as a reinforcement filler. The issues with regard to thermal degradation of cellulose during processing are reduced through the use of specific additives designed to improve its performance during processing. Additionally, other additives are included to improve the UV resistance. Specific combinations of thermal stabilizer and UV additive are provided that enhance the thermal resistance and weathering resistance of the compositions as compared to prior art systems.