Usage of switch mode power supplies (SMPS) has been increased tremendously in recent years due to their advantages compared to conventional ones. In spite of their advantages, SMPSs cause conducted and radiated emissions due to the fact that they switch on and off at specific frequencies. Electromagnetic interference (EMI) filters are mostly preferred equipment for the reduction of conducted emission for coupled circuits. Improved EMI filter in the literature to suppress both common and differential mode noises sourced by ATX (a sample of SMPS) power supply has been proposed. A proposed filter was designed by use of AWR Microwave Office and MATLAB. The power supply was tested according to CISPR22 and it was observed that it has 30 dB higher noise level compared to the limit. A new improvement has also been proposed in order to separate resultant EMI into its common and differential modes for proper EMI filter design. The designed filter, by considering common and differential modes, suppresses those noises by 37 dB.