A model for hysteresis loss of rubber vulcanizates at medium strain (less than 100%) under dynamic condition has been proposed by using Boltzmann superposition principle, statistical theory of rubber elasticity and phenomenological theory. The theory incorporates both experimental and analytical parameters to quantify hysteresis loss. The model with no adjustable parameter has been successfully tested using the experimental results for natural rubber (NR) and styrene-butadiene rubber (SBR) vulcanizates having variations of loading of carbon black, silica, clay, resin and curatives, at various strain rates, strain levels and temperatures. The model is virtually indistinguishable from other well-known models at low strain.