Fatigue　performance　of　butt　welded　splices　in　plates　of　different　thicknesses　(hereafter　referred　to　as　BWSPDT)　was　investigated　in　this　paper.　Fatigue　tests　were　conducted　on　26　specimens.　Experimental　S-N　curve　of　95%　survival　probability　for　BWSPDT　was　established　based　on　21　effective　fatigue　test　data　involving　7　stress　levels.　Both　analytical　and　numerical　fatigue　crack　propagation　analysis　were　fulfilled.　It　was　found　that　the　numerical　approach　would　provide　a　lower　stress　intensity　factor　(SIF)　level　than　that　obtained　through　the　analytical　approach,　as　the　numerical　approach　can　better　reflect　the　relationship　between　the　local　crack　development　and　the　variation　of　the　stress　distribution　of　the　whole　structural　details.　The　influences　on　fatigue　life　due　to　the　variation　of　the　size,　shape　and　location　of　the　initial　crack　were　investigated.　The　calculation　formula　for　the　fatigue　crack　propagation　life　in　terms　of　arbitrary　initial　crack　between　0.075　mm　and　0.4　mm　was　derived.　For　the　fatigue　crack　propagation　analysis　for　BWSPDT,　a　semi-circular　crack　With　0.075　mm　initial　depth　locating　at　the　center　of　the　specimen　was　recommended　to　be　adopted　as　the　initial　crack　input　when　no　detailed　inspection　information　on　initial　flaws　available.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.