The wood plastic composite (WPC) market has grown rapidly. The main advantages of using wood flour or wood fibers as reinforcing fillers in thermoplastics are their low cost, light density, and renewable nature. Due to the hydrophilic nature of wood and the hydrophobic characteristic of polymers, there is poor compatibility between them, and this affects the mechanical properties of the composites. Improving the compatibility and enhancing the mechanical properties has been done by using physical or chemical modification of wood or polymers or by using coupling agents. The potential applications, such as in building industry and furniture, for WPCs have also led to demands for fire resistance. ABS has been categorized into non-charred materials during combustion, and brominated fire retardants (FRs) are commonly used as FRs. However, wood belongs to charred materials during combustion and should act as a halogen-free fire retardant in WPCs. In this study, we investigated halogen and halogen-free flame retardants on the effects of flammability of ABS/wood composites. Ammonium dihydrogen phosphate (ADP) and tetrabromobiphenol A (TBBA) were used as halogen-free and halogenated flame retardants, respectively. Since coupling agents have been widely used to improve the compatibility for WPC, the effect of the combination of coupling agents and fire retardants was also determined. Polystyrene maleic anhydrate (SMA) was used as the coupling agent in our study. Mechanical properties, water absorption, and thermal properties were measured and some of the results are explained through Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) studies. It was found that based on a V-0 rating in a UL-94 type test, fire resistance could be ranked in the order: the combination of both FRs > halogen-free FRs > halogenated FRs in terms of weight loading introduced. With the addition of coupling agents, the fire resistance was improved, but the mechanical properties deteriorated. This may be explained by the fact that coupling agents bring the fire retardants into the ABS phase by hydrogen bonding and the fire retardants act as stress concentrators, reducing the mechanical properties. A similar phenomenon was observed during water absorption measurement. For the samples containing coupling agents, the rate of absorption of water was higher than that of samples without coupling agents. Due to the fact that the driving force for water diffusion into ABS/wood composites is mainly attributed to hydrogen bonding existing in the composites, the presence of fire retardants, which contain the hydrogen bonding, in the ABS phase decreases the barriers to water diffusion. For FTIR study of chars, we examined two situations, ABS/wood composites with ADP and without FRs. From the results, halogen-free FRs catalyzed the dehydration of wood at a relatively low temperature, and this is believed to induce char formation. Due to the fact that the char formation is an exothermic process, we have observed that the released heat accelerated the decomposition of polybutadiene by the disappearance of peak at 910 cm-1. Furthermore, at high temperature, we observed the OH and C=O bonds for ABS/wood composites without FRs, but these two bonds were not detected for the systems with FRs. This may indicate that the char could resist the oxidation better in ABS/wood composites with FRs.