Polycrystalline Cubic Boron Nitride (PCBN) is a super-hard material, which is used in some of the most demanding material removal operations today. PCBN cutting tools are employed in turning of hardened steels as well as machining of other abrasive and aerospace grade alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. Impact loading can lead to the sudden fracture and hence failure of the tool. Much of the recent developments in this industry have been focused on improving the fracture toughness of the PCBN compact. In this work a hypothesis based on induced thermal stresses at the crack tip has been put forward to explain the observed rate sensitivity of PCBN. The results show that the fracture toughness of PCBN is closely linked to the CBN grain size and that the rate sensitivity can be explained in part by induced thermal stresses at the crack tip at high rates of loading.