This paper focuses on waste heat recovery (WHR) system, which is an efficient technology to reduce fuel and vehicle carbon dioxide (CO2) emissions per kW of power produced. Wide variations of power of a vehicle make it difficult to design a WHR system which can operate optimally at all powers. The exhaust temperature from the engine is critical to design a WHR system. Higher the temperature higher will be the gain from the WHR system. However, as power drops the exhaust temperature drops which makes the WHR system perform poorly at lower powers. In this research, a small diesel truck engine was used to design a WHR system to produce additional power using a Rankine cycle (RC). The WHR system was designed at the rated power and speed of 42.8 kW and 2600 rpm, respectively. At this design point, around 15% additional power improvement was achieved resulting around 13% break specific fuel consumption reduction. Next, the performances of the WHR system were evaluated at different operating points lower than the rated power of the engine covering the range of vehicle operations. At few lower power regions, the WHR system could not produce any additional power due to lower exhaust temperatures when designed at the rated power and speed. Then, the WHR system was designed at lower powers which managed to produce additional powers at few lower power regions, but the overall performances of the WHR system were better when designed at the rated power and speed of the engine.