Abstract: ZnO and metal hydroxides as flame-retardants and smoke suppressants for flexible poly(vinyl chloride) were studied.The thermal degradation in air of flexible PVC treated with flame-retardants ZnO and metal hydroxides was studied with differential thermal analysis and thermogravimetry from ambient temperature to 1073 K.The kinetic parameters were obtained by using the equation of Kissinger, Broido and Vyazovkin.Their mechanisms were studied through comparing the thermal behavior with the flame retardant properties of the samples.The results showed that the flame retardant properties of the samples treated with ZnO and metal hydroxides were related to the thermal behavior of the samples.Metal hydroxides enhanced the activation energy of degradation and the flame retardant properties of the samples through endothermic dehydration.ZnO might act in the condensed phase as a Lewis acid.