A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy et al.