The failure mechanism of high-density polybutyleneterephthalate (PBT) was improved by the addition of highly dispersed Fe/FeO particles to the polymer. The impact strength of PBT was measured and the results were used to model the physical structure and failure mechanism of PBT. Fractal analysis was used to investigate the molecular changes caused by the presence of the modifier. The addition of the modifier decreased the free volume, increased the density of the fluctuation network of molecular chains, increased the interphases between crystalline and amorphous phases, and improved the packing of the molecules. The best results were obtained for the addition of 0.05% Fe/FeO.