Energy management is an important portion of controlling total operating costs for refineries throughout the world. For refinery, the crude distillation unit is one of the largest energy consumption units and also represents one of the most important areas for doing heat integration. The heat exchanger network (HEN) of crude distillation units (CDU) can be retrofitted to reduce the utility consumption. This research used the retrofit potential program, Supachai (2011) to find the optimum point in targeting step and then a mathematical programming model using General Algebraic Modelling System (GAMS) called the stage model, using the mixed integer linear programming (MILP) of Yee and Grossmann (1990) was applied to develop the retrofit model and the simulation software (PROII) was used to validate the design and to perform the total utility consumption. With using the existing exchangers, the retrofit model with the exchanger relocation technique is applied. Example problems of HEN for a crude distillation unit with light, medium and heavy crude oil feeds for a period of 100, 150 and 100 days per year, respectively, were used to demonstrate the retrofitting with the aim of finding the optimal design that would yield the highest net present value (NPV).