Three metal hypophosphites, including aluminum hypophosphite (AP), magnesium hypophosphite (MP), and calcium hypophosphite (CP), were applied to flame retard acrylonitrile butadiene styrene (ABS). Thermal stability of three flame-retardant ABS were evaluated, and the enhancement of thermal stability were found. Flammable properties of flame-retardant ABS were investigated by Underwriters Laboratories 94 vertical burning test (UL-94), limit oxygen index (LOI), and cone calorimetry. Results suggested that AP could endow the best flame retardance for ABS with a UL-94 V-0 rating and LOI value of 25.1%. The peak heat release rate of ABS-AP reduced to 174.8 kW/m(2), and the total heat released was decreased to 40.9 MJ/m(2). Thermogravimetric Fourier transform infrared (TG-FTIR), FTIR, and scanning electron microcsopy-energy-dispersive X-ray spectrometry (SEM-EDX) were used to characterize the gaseous products and condensed residue respectively. Results showed that the flame-retardant mechanism was attributed to the formation of a two-layer protective barrier consisting of an organic P-O-C char layer and an inorganic layer to insulate material from fire and oxygen in the condensed phase, and the generation of P-center dot and PO center dot to capture the reactive radicals in the vapor phase.