In this paper, nano-Sb2O3 particles and poly(butylene terephthalate) (PBT) composite powders were dispersed by high energy ball milling method, and then the nano-Sb2O3/PBT composites were prepared by a twin screw extruder. The effects of milling time on the aggregation level and interfacial interactions between nano-Sb2O3 particles and PBT matrix are qualitatively characterized by scanning electron microscopy (SEM), energy dispersive X-ray analyses (EDX) and differential scanning calorimetry (DSC) and quantitatively analyzed by tensile strength and Young's modulus of nanocomposites. The qualitatively results showed that the dispersion and compatibility of nano-Sb2O3 particles within PBT matrix were improved with the increased milling time. From the quantitatively analysis, the aggregation level of nano-Sb2O3 particles within PBT matrix increased with the increased nano-Sb2O3 particles content but it decreased with the increased milling time. Therefore, better dispersed nanocomposites were obtained by the ball milling process which increased the interfacial area between nano-Sb2O3 particles and PBT matrix. As a result, the interfacial interactions between nano-Sb2O3 particles and PBT matrix were significantly improved.