The essential work of fracture (EWF) method was employed to determine the fracture toughness of SAN/PB-g-SAN blends with the compositions of 65/35-0/100. It was found that the plane stress EWF approach is applicable for different SAN/PB-g-SAN blends. During EWF tests, three different types of load-displacement curves were recorded, depending on the blend composition. For the samples containing rubbery phase of 35-65 wt% crack growth occurred immediately after full ligament yielding. Further increase in rubber content and for the samples with 75 and 85 wt% of rubbery phase, the onset of crack growth was preceded by the formation of necking zone in the form of a sharp load drop after full ligament yielding. For the sample with the composition of 0/100, strain hardening behavior was observed without any sign of neck formation. The specific essential work of fracture (EWF) w (e) increased at first with the increase of PB-g-SAN wt% and then decreased with further increasing amount of rubbery phase. The maximum value of w (e) was observed for the blend with 75 wt% of PB-g-SAN. The specific non-essential work of fracture beta W (p) increased with increasing of rubbery phase concentration. The analyzing of yielding and necking/tearing components of essential and non-essential parameters showed that for the samples containing 55 wt% and higher of rubbery phase, and, indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. Finally, it is shown that w (e) values can be predicted via COD values.