Nowadays, there is trend for increasing switching frequency of power devices in order to reduce geometrical dimensions and weight of power converters. High frequency switch mode power supplies (SMPS) generate EMI noise due to their switching actions and parasitic elements which interferes with the normal operation of the other devices or systems. Consequently suppression of EMI is a very important issue in (SMPS) design. In hard switching dv/dt and di/dt are the main sources of EMI noise. In recent years, various methods are used to reduce EMI noise such as grounding, shielding, filtering, passive cancellation. Moreover, it is well known that various softswitching techniques can significantly reduce the dv/dt or di/dt that decreases switching loss and EMI noise. In this thesis, sources of EMI noise, several EMI standards and an EMI measurement method is introduced. Furthermore some of EMI reduction methods in switch mode power supply are discussed. An EMI measurement system in time domain is suggested along with the conventional frequency domain measurement system to extract more information on EMI noise without requiring a noise separator. A Line Impedance Stabilization Network (LISN) is built for measuring the conducted EMI. The measurements are done in accordance with CISPR11 standards. Since understanding and predicting the system EMI noise through an accurate EMI model at the design stage is necessary, thus a precise modeling for active and passive elements and PCB is developed and exact simulation is performed. The experimental results confirm the simulation results. Since softswitching techniques can significantly reduce the switching dv/dt and di/dt, it is conceivable that the EMI noise generated by a hard-switching flyback converter could be reduced by soft-switching techniques. In the last chapter of this thesis the overall performance and the measured results of the conducted EMI emission of a Zero Voltage Transition (ZVT) flyback converter is compared with its hard-switched counterpart. The ZVT converters are PWM controlled and have an auxiliary resonant circuit which is only used during the switching instants to reduce the voltage or current to zero just before switching. The employed ZVT converter provides soft switching condition for all switching elements. This reduces the amount of high frequency harmonics generated by high dv/dt and di/dt . Keywords: Electromagnetic Interfrence (EMI), Flyback Converter, Conducted EMI Measurement, Soft Switching, Zero Voltage Transition (ZVT)