The rubber particle size and its volume fraction are recognised as being important factors in determining the yield and fracture behaviour of high impact polystyrene (HIPS). However, correlations between the average particle size and cavitation in the rubber with toughening efficiency have only recently been established theoretically. This work provides further evidence on how the deformation kinetics in HIPS are affected by variations in the average rubber particle size highlighting along the way the role of rubber cavitation in the process. Variations in the average particle size were achieved by melt blending different proportions of two commercial grades of HIPS that had the traditional multiple inclusion particle morphology. Tensile and impact properties of the blends were measured and correlated to morphological parameters determined by quantitative image analysis. It was found that yield and fracture behaviour in tensile and impact test were strongly dependent on the amount of sub-micron particles in the blend. At high rates, toughness drops steeply with particle size. It was proposed that stress at yield and post yield strain hardening are controlled by particle size and rubber stretching respectively. Microfracture analysis by transmission electron microscopy lent support to the arguments presented.