A steam plant layout for the Waste Heat Recovery (WHR) of a marine four-stroke Dual Fuel (DF) engine is analysed by numerical simulation. The different values of exhaust gas mass flow and temperature between the two fuel modes involve different Rankine cycle thermodynamic characteristics, and therefore different heat exchangers sizes in order to obtain the greater waste heath recovery from the engine in both its operating modes. The two developed WHR solutions, optimized for each fuel type, are compared for the normal working conditions of the engine. In particular, the comparison is focused on the maximum plant efficiency and on the size and weight of the Heat Recovery Steam Generator (HRSG) components. Finally the best WHR compromise solution is indicated and tested for different load conditions and fuel types of the considered DF engine.