The Gain flatness of EDFA plays an important role for Wavelength Division Multiplexing (WDM) optical application. Equalizing of an amplifier's gain spectrum is essential for balancing the channel powers to achieve error free detection of the signals transmitted through the optical link. The purpose of this paper is to improve the gain unevenness for each channel in order to equalize the amplitude in a Wavelength Division Multiplexing (WDM) system. And also various filters have been taken under study to find its effect on the output. The design simulation aided by Optisystem 12 software. From the analysis it has been found that the gain flatness is improved by optimizing the pump power, pump wavelength and fiber length. The optimized value of EDFA to achieve maximum gain flatness under different filters are presented. The gain are flattened within 32.75dB-33.67dB from 1546 nm-1558 nm band of wavelength with noise figure (NF) < 7dB, output power 15 dB, Bit Error Rate (BER) < − & Quality factor is 14.64. By these above parameters, gain flatness of 0.3dB can be achievable.