The kinetics of cementite coarsening in tempered martensite AISI 4340 steel system has been investigated in order to understand over-tempering behavior. Microstructural response during tempering at 550 and 650 A degrees C has been recorded, analyzed, and modeled. A statistical analysis of metallographic sections was performed to yield measurements such as average particle size, volume fraction, morphology, volume number density. The coarsening behavior of this multicomponent alloy system has been modeled with the classical coarsening theory for multicomponent alloys. These results highlight the remarkable coarsening resistance of 4340 steel even for prolonged tempering.