Abstract: The theoretical critical impact velocity in tension is analyzed. It is indicated that the rate-dependent Karman-Taylor theory of plastic wave propagation, the complete thermal coupling constitutive relation at high strain rates and the equation of energy balance applicable to adiabatic heating are essential to determining the theoretical critical impact velocity in tension. For oxygen-free high-conductivity copper, the theoretical critical impact velocities in tension are calculated with two typical constitutive relations at high strain rates. The numerical results show that the theoretical critical impact velocities in tension calculated with Johnson-Cook constitutive relation are smaller than that calculated with Zerilli-Armstrong constitutive relation. It is also noted that the theoretical critical impact velocity in tension is defined by the plastic wave trapping, while the experimental critical impact velocity in tension is determined by the wave effect, by the necking phenomenon and by the micro-damage evolution of a high-velocity tension bar.