Design models of gas-liquid stirred tank reactors, for both cases of continuous and semibatch operation, are developed in this paper based on the penetration theory. The penetration concept of mass transfer accompanied by chemical reaction have mathematically been treated in a new form to describe the gas-liquid systems conviently. Analytical or pseudo-analytical solutions have been obtained for the design equations, with that one can easily calculate the concentrations in the both phases, mass transfer degree and reaction conversion.