In this paper, nano-Sb2O3 particles and poly( butylene terephthalate) (PBT) powders were dispersed by high speed rotating and high energy ball milling methods, and then the PBT/nano-Sb2O3 composites were prepared by a twin screw extruder. The agglomeration level and interfacial properties between nano-Sb2O3 particles and PBT matrix are quantitatively analyzed by the tensile strength and Young's moduli of the nanocomposites. The calculated results show that the agglomeration level of nano-Sb2O3 particles in the PBT matrix increases with increasing the nano-Sb2O3 particles content. Because the agglomerated nano-Sb2O3 particles reduce the interfacial area in the PBT matrix, the interfacial adhesion sharply decreases at the high nano-Sb2O3 particles content. In addition, the nano-Sb2O3/PBT composites dispersed by the HEBM process have better interfacial properties due to the lower agglomeration level than that of HSR.