The paper presents a theory on combustion similarity in diesel engines and contributes to the realization of scale-model experiments. The theory is based on the correspondence of fundamental equations of change and the correspondence of both initial and boundary conditions for different-sized engines. The equations of change consist of continuity, motion, and energy equations, which apply for phenomena with combustion-induced flow and density change. As a result of the investigation, it was found that there exists a possibility of combustion similarity for different diesel engine sizes. The similarity conditions identified involve engine configuration, injection system, fuel properties, swirl ratio, engine peed, droplet size of the fuel spray, temperature of the cylinder wall, and ignition lag.