This work deals with a thermodynamic analysis of crude oil distillation systems to study energy and exergy efficiencies for system analysis, performance evaluation and optimization. In this regard, a new analysis methodology is proposed, and the SimSci/PRO II ( 2000) is employed as a simulation package for the required parametric studies, particularly to study the effects of the distillation column temperatures and pressures on the efficiencies. The simulations are carried for the operating conditions of the model. It is found that the total irreversibility losses are 608 MW for a flow rate of 507 kg s Γ1 [e.g. 56% by the Atmospheric Distillation Unit (ADU), 26% by the Vacuum Distillation Unit (VDU) and 18% by the heaters]. The highest irreversibility losses occur in the ADU as the main separation takes place there. Of the losses, 6.2% are due to chemical exergy losses associated with the separation process itself. The rest of the losses are due to the physical exergy losses mainly because of the temperature difference. The energy efficiencies are 0.497 for the ADU, 0.579 for the VDU and 0.519 for the overall system. The exergy efficiencies are 0.433 for the ADU, 0.501 for the VDU, 0.821 for Heater 1, 0.956 for Heater 2 and 0.233 for the overall system. The overall efficiency is not equal to the product of the components efficiencies.