In heavy duty diesel engines, the waste heat recovery has attracted much attention as one of the technologies to improve fuel economy further. In this study, the available energy of the waste heat from a high boosted 6-cylinder heavy duty diesel engine which is equipped with a high pressure loop EGR system (HPL-EGR system) and low pressure loop EGR system (LPL-EGR system) was evaluated based on the second law of thermodynamics. The maximum potential of the waste heat recovery for improvement in brake thermal efficiency and the effect of the Rankine combined cycle on fuel economy were estimated for each single-stage turbocharging system (single-stage system) and 2-stage turbocharging system (2-stage system). As a result of estimation, the maximum brake thermal efficiency by the waste heat recovery is theoretically 55.0 % (single-stage turbocharging system) and 53.3% (2-stage turbocharging system) at full load operating condition (Ne=1600rpm), if the available energy of the waste heat in the high boosted diesel engine is converted into the effective work unless considering actual losses. On the other hand, the conversion efficiency from the waste heat to effective work by using the Rankine combined cycle was predicted fairly low. The estimated improvement in fuel consumption by the Rankine combined cycle was 2.7% in single-stage system and 2.9% in 2-stage system, on the condition of the highway cruising at constant speed of 80km/h with a full payload for the heavy duty vehicle (GVW=24980kg).