Bearing　fault　diagnosis　that　utilizes　motor　current　has　shown　great　potential　for　industrial　application.　For　estimating　fault　sizes,　this　paper　develop　an　electromechanical-magnetic　coupling　computational　model　of　induction　motor　(IM)　based　on　modified　winding　function　approach　(MWFA)　in　order　to　extract　the　fault-excited　harmonic　components　and　amplitudes　in　stator　current.　A　new　evaluating　indicator　FHD　(Fault-excited　Harmonic　Distortion)　is　then　proposed　to　describe　the　specific　relation　between　fault　sizes　and　severity　of　fault-excited　harmonic　distortion.　The　statistical　results　show　that　there　is　a　fixed　functional　relation　curve　between　FHD　and　fault　sizes　in　different　IM.　Finally,　an　experimental　IM　with　a　known　size　built-in　faulted　bearing　is　investigated.　The　1　mm　artificial　damage　on　outer　raceway　is　recognized　by　calculating　FHD　(0.5636　in　average)　value　and　comparing　with　the　simulated　relation　curve.　The　results　validate　the　accuracy　and　robustness　of　the　proposed　approach　in　fault　size　estimation.　©　2020