In order to reduce vehicle weight and improve crash performance, dual-phase (DP) steels are increasingly used for lightweight auto-body. the high cooling rates in Resistance spot welding (RSW) process will make the nugget very brittle and result in interfacial fracture modes. In this study, a servo resistance welding system is firstly introduced to control process parameters during the welding process. The influences of different process parameters (weld current, weld time and electrode force) on the weld strength, nugget diameter, facture modes and weld indentation are analyzed. Finally, an on-line weld failure mode inspection method is proposed based on servo gun’s feedback characteristics.