Optical MEMS devices rely on the micro assembly to achieve re-positioning, such as lifted up micro mirrors and lens and micro resistance welding benefits assembly of optical components. However, the characteristics of micro resistance welding are still unknown. The purpose of this study is to characterize micro resistance welding with electro-thermal actuator for micro assembly. In order to characterize influence of operation parameters on micro resistance welding, important parameters including contact pressure, contact resistance and electrical energy are calibrated. Further, welding strength provide robust join are also measured. The idea of resistance welding is based on generated heat by ohm's law to melt material. From measured results, contact resistance decreases with increasing contact pressure due to increasing contact area. The stronger welding strength can be achieved at a smaller initial contact resistance which means that a larger clamping force could enhance the welding strength. The maximum welding strength is 74.4 μN at 2.7 Ω. Further, welding energy affects yield significantly. At high welding energy, between 1 to 10 J, the yield can reach 100 %. The energy below 0.05 J would not generate adequate heat to weld structure.