As oil crisis and environment pollution problem become increasingly prominent, gaseous fuel internal combustion engines (gas engines) have been valued more and more because of the great resources potential and the feature of low pollution of gaseous fuel. However, their average efficiency is just about 30-40% wasting large amount of energy by exhaust, cooling water and so on. Therefore, it is an effective way of improving the efficiency to recover waste heat. Both the RC (steam Rankine Cycle) and the ORC (Organic Rankine Cycle) are considered as quite suitable methods of WHR (waste heat recovery), especially for large engines. As the WHR system for a specific large gas engine, RC and ORC both have advantages and disadvantages. A gas engine of rated power 1000 kW is the studied object in this paper. According to the characters of its waste heat, the WHR systems of RC and ORC with different working fluids, including a two-stage Organic Rankine Cycle (DORC) are built in math model and they are compared from the aspects of recovered power, thermal efficiency, system feasibility and so on, which provides a reference for the engineering application. Finally, the economic performance of ORC, RC and DORC are evaluated and compared. (C) 2016 Elsevier Ltd. All rights reserved.