Eddy　current　testing　(ECT),　as　a　noncontact　and　nondestructive　testing　(NDT)　technology,　has　been　widely　applied　to　various　industrial　fields　for　pipeline　defect　detection.　In　this　article,　a　combined　defect　parameter　estimation　method　using　a　novel　arc-shaped　differential　probe　with　orthogonal　receivers　is　proposed　to　detect　circumferential　defects　in　pipelines.　The　sensitivity　between　coils　is　analyzed　to　optimize　the　probe　parameters.　The　scanning　signals　of　defects　on　the　complex　plane　are　characterized　by　the　shell　curves　(SCs),　and　a　Lissajous　curve　(LC)　model　is　employed　for　curve　fitting.　It　is　observed　that　a　monotonous　relationship　holds　between　fitting　parameters　and　defect　dimensions.　From　the　fit　scanning　curve,　the　defect　depth　and　length　can　be　estimated　by　employing　the　K-nearest　neighbors　(KNNs)　algorithm.　Experiments　have　been　conducted　to　verify　the　proposed　defect　parameter　estimation　method　by　testing　aluminum　pipelines　with　cracks.　The　experimental　results　show　that　the　estimation　of　defect　parameters　achieves　an　average　accuracy　(ACC)　of　more　than　96.78%　for　various　dimensions　of　cracks.　©　1963-2012　IEEE.