The　quality　of　steel　balls　influences　the　accuracy,　motion　performance　and　service　life　of　the　rolling　bearing.　The　eddy-current　testing　technique　plays　an　important　role　in　the　measurement　of　bearing　ball　properties,　including　radius,　electrical　conductivity　and　relative　magnetic　permeability.　In　this　study,　an　estimation　method　of　bearing　ball　properties　is　proposed　using　the　multi-frequency　eddy-current　testing　technique.　The　estimation　method　optimizes　the　least　squares　problem　between　the　measured　and　calculated　coil　inductance　spectra　through　the　modified　Newton-Raphson　algorithm.　In　addition,　the　combination　of　bearing　ball　properties　to　be　estimated　is　evaluated　by　employing　sensitivity　analysis.　The　effectiveness　of　the　proposed　method　has　been　evaluated　by　numerical　simulation　and　experiments　measuring　bearing　balls　with　various　properties.