The　electric　coupling　between　primary　and　secondary　windings　of　the　transformer　is　usually　evaluated　by　the　capacitance　between　the　primary　and　secondary　windings　which　is　tested　by　one-port　test　devices,　such　as　LCR　meter.　This　method　can＇t　completely　characterizes　the　EMI　behaviors　of　the　transformer　on　common-mode　noise,　because　this　method　does　not　account　electrical　potential　distribution　along　the　winding　turns　and　the　shielding　copper　foil　effect　in　the　transformer.　A　new　evaluation　method　is　proposed　in　this　paper　which　can　overcome　the　traditional　shortages　on　CM　noise　by　spectrum　analyzer,　EMI　receiver　or　network　analyzer.　The　method　provides　an　effective　test　tool　to　optimize　the　structures　of　the　windings　and　shielding　of　a　transformer　in　design　and　to　control　the　quality　of　the　transformer　in　mass　production　with　its　CM　EMI　behaviors.　Finally,　the　experimental　results　verify　the　proposed　method　to　be　correct　and　feasible.　©　2011　IEEE.