Abstract: Abstract: In recent years, plant fiber reinforced thermoplastic composites have been widely used as interior automotive panels, garbage pails, crates, and garden equipment due to their excellent durability, dimensional stability, high rigidity, and relatively low density. However, its poor fire resistance often limits its expansion into the residential construction industry and development of applications for the furniture industry. In order to improve the flame retardancy of the reinforced composites, fire retardant agents, such as halogen/nitrogen-containing substances, metal hydroxides, and phosphorus/silicon compounds, have been introduced. While there are environmental impact concerns, the most commonly used flame retardant additive is ammonium polyphosphate (APP). Recently, the synergistic effect between nano-SiO2 and ammonium polyphosphate (APP) in polymer and composites has been extensively investigated, and study has showed that the novel nano-SiO2/APP flame retardant could promote char formation, and improve the thermal stability of the char layer formed during combustion.China has been an important agricultural nation with 300 million tons of rice production annually. Using rice straw as a substitute feedstock material of wood fiber could be helpful for conserving valuable wood resources, protecting the environmental and contributing to sustainable development. Rice straw consists of cellulose, hemicellulose, lignin, silicon and pectin. Silicon, which is widely distributed in the surface of rice straw through biomineralization, could be also used as a synergistic agent of APP. In this paper, rice straw-high density polyethylene composites (RPC) with ammonium polyphosphate (APP, 0, 8%, 10%, and 12%) were prepared to investigate the effects on the thermal properties, flame retardancy, and mechanical properties of RPC using thermogravimetric analysis (TG), cone calorimentry analysis (Cone) and mechanical measurements. The mechanism of flame retardancy are also investigated with scanning electric microscopy/x-ray energy dispersive spectroscopy analysis (SEM-EDXA).1) TG results showed that: RPC had a lower initial temperature (Tinitial), and peak temperature of thermal degradation (Tpeak) when compared to wood-high density polyethylene composites (WPC). Furthermore, RPC possessed higher char residues. Introducing APP to RPC, Tinitial was shifted to a lower temperature of 8-9℃, that means APP made the thermal degradation of RPC take place earlier. APP also increased Tpeak and improved the char residues. 2) Cone results showed that RPC exhibited better flame retardancy, and the incorporation of APP to RPC could improve the flame ratardancy. The lowest heat release rate (HRR) and total release rate (THR) both occurred when the APP addition reached up to 12%, with corresponding to a decrease of 20.4%, 20.7% and 11.0% in aHRR, pkHRR, and THR respectively, and an increase of 12.7% in the limited oxygen index (LOI). 3) The mechanical properties showed that RPC possessed better toughness, and the APP addition had a positive effect on toughness, while it had no appreciable effect on the flexural and tensile strength of RPC.