The　effects　of　strength　mis-matching　on　J-integral　and　failure　assessment　line　(FAL)　of　electron　beam　welded　thick　TC4-DT　titanium　alloy　joints　have　been　investigated.　Elastic　and　elastic-plastic　finite　elements　(FE)　calculations　on　centre　crack　tensile　(CCT)　specimens　were　made,　and　the　condition　of　20%　strength　mis-matching　was　considered.　The　results　indicate　that　the　limit　load　of　the　joint　can　be　substituted　by　that　of　the　base　metal.　The　error　of　substitution　is　less　than　5%.　The　failure　assessment　lines　for　both　over-matching　and　under-matching　joints　are　increasingly　close　to　that　of　the　base　metal,　with　the　ratio　of　weld　width　to　specimen　width　(H/W)　decreasing.　Moreover,　the　failure　assessment　lines　of　the　joints　can　be　devised　using　the　stress-strain　curves　of　the　base　metal,　which　will　be　convenient　for　the　practical　application　of　engineering　structures