Martensitic　stainless　steels　exhibit　poor　resistance　spot　weldability,　with　high　susceptibility　to　interfacial　failure　due　to　the　formation　of　brittle　martensite　in　the　fusion　zone　(FZ).　To　alter　the　composition　of　the　FZ　and　improve　the　lap-shear　performance,　Ni　interlayer　was　employed　during　the　resistance　spot　welding　(RSW)　of　2-mm-thick　630　martensitic　stainless　steel　(MSS).　The　microstructures　of　the　joints　with　and　without　Ni　addition　were　characterized　using　optical　microscopy　and　EBSD　analysis.　Calculation　of　phase　diagrams　(CALPHAD)　technique　was　used　to　analyze　the　phase　evolution　in　the　joints　as　a　function　of　temperature.　The　FZ　of　the　joint　without　Ni　addition　consisted　of　predominantly　martensite　with　small　amount　of　dispersed　retained　austenite.　Under　lap　-shear　loading,　the　crack　propagated　through　the　martensitic　FZ,　leading　to　interfacial　failure　(IF),　irrespective　of　the　welding　time　or　current.　The　joints　could　only　reach　a　peak　load　of　8.26　kN　and　energy　absorption　of　6.83　J.　The　Ni　addition　changed　the　solidification　mode　from　ferritic-austenitic　to　fully　austenitic　mode　and　signifi-cantly　lowered　the　Ms.　temperature　(from　108　to-188.7　degrees　C).　Accordingly,　the　Ni-added　joints　exhibited　fully　austenitic　microstructure　and　failed　largely　through　pullout　mode　with　significantly　improved　energy　absorption.　They　exhibited　a　peak　load　of　13.27　kN　and　energy　absorption　of　27.22　J.