The recently developed compact compression specimen (CCS) — H integral technique is applied to the characterization of the dynamic fracture toughness of PMMA under transient loading. The forces and displacements on the boundaries of a cracked CCS are applied and determined using a Kolsky apparatus. The path-independent H-integral is thus calculated by forming a convolution product between experimental and reference data. The evolution of both the mode I and mode II stress intensity factors is determined by solving linear convolution equations. Therefore, the history of the stress intensity factors is assessed from the onset of loading until early crack propagation detected by a fracture gage. Dynamic fracture toughness is taken as the value of the mode I stress intensity factor at fracture time. Experimental results compare well with previously reported values while extending the range of applied loading rates. The fracture toughness is observed to increase markedly with the stress intensity rate. This observation is discussed in the light of fractographic examination showing the existence of a characteristic rough zone directly ahead of the notch-tip of dynamically fractured specimens.