Physical, mechanical, and fire properties of the flat-pressed wood plastic composites (WPCs) incorporated with various fire retardants (10% by weight) at different levels of wood flour (WF) content, 40, 50, or 60 wt%, were investigated. The WPC panels were made from dry-blended WF, polypropylene (PP), and fire retardant (FR) powders with maleic anhydride-grafted PP (2 wt%) formulations using a conventional flat-pressing process under laboratory conditions. Incorporation of the fire retardants into the WPC panels significantly decreased the internal bond strength compared to the WPC panels without FR at all levels of the WF content. The modulus of rupture of the WPC panels containing FRs decreased with the increase in the WF content from 40 to 60 wt%. The modulus of elasticity increased with the increase in the WF content from 40 to 50 wt% and then decreased as the WF content reached 60 wt%. The WPC panels incorporated with zinc borate gave an overall best performance in both physical and mechanical properties followed by the panels treated with decabromodiphenyl oxide, magnesium hydroxide, and ammonium polyphosphate. Higher levels of the WF content resulted in significantly improved fire resistance of the WPC panels with and without FR as measured in the cone calorimeter. Of the four fire retardants tested, ammonium polyphosphate showed the most improvement over untreated ones.