This paper presents the simulation methodology for the buffer size determination in FMS(Flexible Manufacturing System) cell line which is categorized into cell buffer and machine buffer. The simulation model is designed a new methodology for developing FMS design in an Aspect-oriented environment. Aspect-oriented approach provides a new way of thinking about FMS simulation design. Using the evolution strategy, the solution of the objective functions were identified for the optimal buffer sizes in the proposed FMS cell line. The buffers play important roles in dynamical system situation, so efficient buffer size determination could improve system performance. The merits of the aspect-oriented concept such as the clear responsibility of each module, code re-use and effective implementation of functions allow for designing an FMS simulation module effectively.