By adopting the method of melt blending, with an addition of MMT and ammonium-phosphate-based flame retardant to PHBV/PBAT composites, a research has been carried out on the combustion performance, mechanical and rheological properties of PHBV/PBAT composites. Results concerning the limiting oxygen index and vertical burning show that the ammonium-phosphate-based flame retardant helps to improve the limit oxygen index of the PHBV/PBAT composites, and the addition of MMT contributes to a further improvement of the limit oxygen index, followed by the V-0 level of UL-94 vertical combustion tested, with MMT 1% of the content. Experimental results of the rheological and mechanical properties show that the ammonium-phosphate-based flame retardant deteriorates the mechanical properties of the composites, while MMT helps to improve the dispersion properties of the powder in matrix materials, as well as the mechanical properties of the flame retardant composites.The cone calorimeter test shows that an addition of montmorillonite significantly decreases the heat release rate and the amount of smoke production of the composites.