Resistance welding is a common scheme of assembly on the macro scale by pressing together two workpieces with current passing through them to generate joule heating at the contact region due to high contact resistance. However, micro assembly by resistance welding is seldom reported. Here, resistance welding with an electro-thermal microactuator to assemble micro Ni structures is experimentally investigated and characterized. The bent-beam electro-thermal microactuator is designed to provide the necessary displacements and pressing forces. The two-mask metal-based surface micromachining process is adopted to fabricate the micro Ni structures. The calibrated initial contact resistance is shown to decrease with increasing contact pressure. Furthermore, stronger welding strength is achieved at a smaller initial contact resistance, which indicates that a larger clamping force would enhance the welding strength as large as 3.09 MPa (74.4 µN) at a contact resistance of 2.7 Ω here. The input welding energy is also found to be a critical factor. In our tests, when welding energy is below the threshold limit of 0.05 J, the welding trials all fail. For the energy between 0.05 J and 1 J, there is a transition from a lower yield of 33.3% to a higher yield of 58.3%. At high welding energy, between 1 and 10 J, 100% yield is achieved. With the demonstration and characterization of micro resistance welding by the electro-thermal microactuator, the scheme proposed here would be helpful in the automation of micro assembly.