The　electric　coupling　between　the　primary　and　secondary　windings　of　the　transformer　is　usually　evaluated　by　the　capacitance　between　the　primary　and　secondary　windings　with　LCR　meter.　This　method　cannot　effectively　consider　the　shielding　copper　foil　effect　and　the　electrical　potential　distribution　along　the　winding　turns　in　the　transformer　in　characterizing　the　EMI　(electro-magnetic　interference)　behaviors　of　the　transformer　on　common-mode　noise.　In　this　paper,　the　coupling　was　analyzed　according　to　the　electromagnetic　theory.　Based　on　the　analysis,　a　new　evaluation　method　was　adopted　by　two-port　equipment,　such　as　spectrum　analyzer,　network　analyzer　and　EMI　receiver,　which　can　consider　the　effects　of　shielding　foil　and　potential　distribution　along　winding　turns　on　common-mode　(CM)　noise.　Finally,　the　experimental　results　verify　the　method　to　be　correct　and　feasible.　The　method　provides　an　effective　test　tool　to　optimize　the　electromagnetic　compatibility　(EMC)　design　of　the　transformer　and　to　control　the　quality　of　the　transformer　in　mass　production　with　its　CM　EMI　behaviors.　©　2012　Chin.　Soc.　for　Elec.　Eng.