The shear strength of pure nickel (Ni), and its alloy, Ni-60Co (by wt%), has been determined during one-dimensional shock loading in the impact stress range 0-10 GPa. The influence of the reduced stacking fault energy (SFE) for the Ni-60Co has been investigated. The shear strength (tau) and the lateral stress (sigma(y) ) both increase with the impact stress for each material. The shear stress has been found to be higher in the nickel than in the alloy. The progressive decrease of the lateral stress behind the shock front indicates an increase of the shear strength. A more complex mechanism of deformation has been found for the alloy since twin formation has been observed in the microstructure, while none has been seen in nickel. It is thought that mechanical twinning plays a predominant role in the deformation mechanism of the alloy resulting in the reduction of the material strength.