The objective of this research was to improve the fire-retardant properties of wood in one treatment using a phosphorus pentoxide–amine system. Phosphorus pentoxide and 16 amines including alkyl, halophenyl, and phenyl amines were compounded in N,N-dimethylformamide and the resulting solutions containing phosphoramides were reacted with wood. The characteristics of phosphoramide-reacted wood were analyzed by FTIR, energy-dispersive X-ray analysis, and elemental analysis. Fire retardancy of phosphoramide-reacted woods was evaluated by DSC and thermogravimetric analysis (TGA). DSC demonstrated that the modification can lower the onset of an endotherm and decrease the heat of combustion and heat flow. TGA showed that most of the phosphoramide-reacted woods had higher char yields than that of wood impregnated with diammonium phosphate.