Abstract: Flame retardant poly(lactic acid)/bamboo flour composites were prepared with aluminium hydroxide（ATH）, ammonium polyphosphate (APP), and APP/ATH blends as a flame retardant. Water absorption and mechanical properties of APP/ATH blends were tested after being soaked in water for various length of time. The tensile strength and impact strength properties of composites were decreased after being the soak treatment, in which the tensile strength decreased by nearly 90 % after being soaked 4 d. The blend containing either APP or ATH alone had low water absorption and thickness swelling rate，and those containing both APP and ATH provided the highest water absorption and the worst dimensional stability.