In heavy duty diesel engines, waste heat recovery systems are remarkable means for fuel consumption improvement. In this paper, Diesel-Rankine combined cycle which is combined diesel cycle with Rankine cycle is studied to clarify the quantitative potential of fuel consumption improvement with a high EGR rate and high boosted diesel engine.The high EGR rate and high boosted diesel engine of a single cylinder research engine was used and it reaches brake specific fuel consumption (BSFC) of 193.3 g/kWh at full load (BMEP=2.0MPa). And its exhaust temperature reaches 370 C. The exhaust gas temperature does not exceed 400 C in high boosted diesel engine even at full load operating condition because of a high excess air ratio. On the other hand, exhaust gas quantity is larger due to a high boosting. So, it is estimated that the thermal energy of exhaust gas is enough for recovery in the high boosted diesel engine, although exhaust gas temperature is not so higher than that of an ordinary diesel engine.In the heat balance of the high boosted research diesel engine at medium engine speed, the exhaust loss is 38 % at full load. From this result, it is possible to recover the exhaust gas energy, when engine is operated above medium load condition. In this predictive study, water, methanol, toluene, HCFC-123, R134a and R245fa are compared as working fluid in Rankine cycle with superheating. As a result of this study, it is found that Diesel-Rankine combined cycle has a potential to improve BSFC for 2.6 - 3.0 % at full load condition.