Resistance projection welding (RPW) of T-shaped nuts to Usibor1500 hot-stamping steel sheets was conducted using Capacitor Discharge Welding (CDW). The microstructure evolutions, failure modes, and failure mechanisms of the RPW joints were investigated. The results show that the microstructure of the joints is divided into three regions: the fusion zone (FZ), the heat affected zone (HAZ), and the base material (BM). Predominant HAZ softening occurs on the HAZ of the sheet side due to the formation of ferrite and tempered martensitic. There are three modes of RPW joint failure: interfacial failure (IF), pullout failure (PF), and projection failure. Due to the uneven distributions of tensile stress on the faying surface, the fracture of the joints in the IF mode is a combination of ductile and brittle fracturing. The crack of the PF mode is generated in a ductile manner, but propagates around the joint in a brittle way. The fracture morphology of the projection failure mode is of large cleavage faces and river patterns. The solidification crack around the shrinkage void changes the crack propagation path and leads to a brittle rupture. As the welding voltage increases, the joints fail in mix modes: the pullout and interfacial failure mode or the pullout and projection failure mode.