When a shock wave, which can be generated by high velocity impact or explosive detonation, reflects from the free surface of a metal it usually creates tensile stress inside the metal. While the tensile stress is large enough, voids nucleation, growth and coalescence happen inside the metal, causing spallation of the metal. As one of the main contents of the spall damage research, the spall strength, which is often characterized by features of the free surface velocity history measured in spall experiments, represents the maximum tensile stress that the material can withstand and is actually a complex interaction of several competing mechanics. Optimizing the spall strength of metals is important to their utility within the aerospace, automotive, and defense industries, and can be achieved using advanced manufacturing strategies, if we can better know the meaning and present analytic model of the spall strength of metals. A large number of experiments show that the spall strength of ductile metals is strongly dependent on the tensile strain rate, grain size and temperature of material. Based on the analysis of early spall evolution and influence of grain size and temperature on the material, an simple analytic model of spall strength is presented in this paper, which accounts for effects of strain rate, grain size and temperature in materials. The application of this model is verified by comparing with the experimental results of spall strength of typical ductile metals such as high purity aluminum, copper, and tantalum.