The present paper describes an investigation into the effects on the fatigue behaviour of adding up to 1.00 vol.% ZnO nano-rods with a diameter of 13 nm and an aspect ratio of three to two amorphous styrene acrylonitrile copolymers. Two acrylonitrile contents, i.e. 24% and 34% respectively, were studied. Fracture mechanics based fatigue tests were conducted at 5 Hz and a Paris Law analysis approach was followed. The fatigue threshold increased notably with the addition of the ZnO nano-rods. The increases in toughness were analysed using an analytical model of plastic void growth around the particles and this enabled the surface energy per unit area for the particle-matrix debonding process to be deduced. Good particle distributions were achieved only for very small volume fractions (<0.3%) above which the occurrence of agglomerated particles and consequential toughness declines were observed. In the fatigue threshold region, surface micrographs showed clear evidence of debonding and plastic void growth and the average measured void diameter agreed closely with that predicted by the model.