Supercritical CO2 (S-CO2) cycle systems have emerged as an attractive alternative for internal combustion engine (ICE) waste heat recovery thanks to the advantages offered by CO2 as a working fluid , incl uding robust performance and system compactness. The engine exhaust gases are the main available heat source from ICEs with promising thermodynamic potential for further utilisation, and whose conditions, i.e., temperature and mass flow rate, vary based on the ICE operating strategy load. These heat source variations have a critical influence on the performance of a bottoming S-CO2 cycle system, which needs to be carefully considered in the design stage. This paper explore s the optimal design of S-CO2 cycle system s for ICE waste heat recovery considering heat source fluctuations as well as the probability of their occurrence as arising from actual ICE operation. A variety of heat source conditions are selected for separate design s of an S-CO2 cycle system and performance prediction under all possible scenarios is evaluated via detailed design and off design models, so as to select the optimal design that is able to match the heat source fluctuations and exhibit the best performance from thermodynamic and economic perspectives. The advantage of this approach relative the conventional ones that only consider one specific design condition is that it avoid s either over or under sizing of the S-CO2 cycle system, which also achieves comprehensive insight of the interplay between the bottoming heat recovery system and the ICE, and provides valuable guidance for further system optimisation.