A wavelength division multiplexing (WDM) network are widely used in modern telecommunication infrastructure, which are expected to support a large variety of services with different requirements in terms of latency, bandwidth, reliability and many other features. The gain flatness of Erbium Doped Fiber Amplifier (EDFA) plays an important role in WDM network. The main drawback of EDFA is its unequal gain spectrum. The purpose of this project is to investigate different gain flatness techniques for EDFA in order to optimize the WDM performance, as well as to achieve a given bit error rate (BER), gain flatness and noise figure of EDFA through optimized fiber length and pump power. A commercial software, known as ¿Optisystem¿ is used in this project for implementing the physical model of WDM system with different gain flatness techniques. At the end, the results obtained from the simulation of WDM systems implemented with different gain flatness techniques are compared with each other.