In this study, a 1.1 MW diesel-gen-set is used to design a Waste Heat Recovery (WHR) system to generate additional power using Rankine cycle (RC). A computer code is written in commercial Engineering Equation Solver (EES) software to solve equations of overall energy and mass balance, heat transfer, evaporation, condensation, frictional and heat losses for heat exchangers, turbine, pumps, cooling tower and connecting pipes connecting different components. After initial design of the WHR system, manufacturers are contacted to find out the availability of parts, and then, accordingly the design is changed. There are several heat exchangers required to heat the water from liquid to superheated steam and then, it is passed to the turbine. Then, after the expansion in the turbine, it is passed to the condenser to condense the steam to water. Optimization is done on the heat exchangers, focusing on the tube length and diameter. The tube length is changed in accordance to the availability on the market, where it comes in 2 m length. At the rated power of the gen-set, with a pressure ratio of 100 (inlet and outlet pressures to the turbine are 30 bar and 0.3 bar, respectively), an overall improvement of 12.2% is achieved. Results from this study can be applied to large diesel engines used for large trucks which fall in the USA commercial truck category of Class 7 and 8 of heavy trucks, large diesel-gen-set and mobile applications such as ships, heavy construction equipment, ultra-heavy mining movers, excavators and locomotives. It found that the WHR system could be as heavy as the gen-set and they are bulky. Therefore, not only the overall efficiency improvement is important, the size and weight of the WHR system is also very important.