In this study, a micro tensile test method that can measure the interface strength of a grain boundary has been developed by applying an EBSD (Electron Back-Scatter Diffraction) method and a FIB (Focused Ion Beam) system, and it was applied to evaluate the effect of the crystallinity of a grain boundary on the strength of electroplated copper thin films quantitatively. The position and crystallinity of a grain boundary in a polycrystalline electroplated copper thin film were preliminarily determined by EBSD method, and the micro scale test specimen was cut out from the appropriate area in the film by using FIB. Therefore, a bicrystal sample which consisted of the characterized single grain boundary was cut from a polycrystalline thin film, and the strength of one grain or one grain boundary was measured quantitatively. In this study, the crystallinity of grains and grain boundaries was evaluated by using Image Quality (IQ) value obtained from the EBSD method. As a result, the fracture mode and strength of the polycrystalline copper thin films were found to vary drastically depending on the crystallinity of the grain boundary. The specimens including a grain boundary with average IQ value lower than 3500 showed brittle fracture at the grain boundary. On the other hand, in the specimens with average IQ value higher than 3500 showed ductile transgranular fracture. In addition, it was confirmed that the strength of a grain boundary with average IQ value lower than 3500 decreased with decreasing the IQ value and the yield strength of a grain decreased with increasing the average IQ value of a grain. It is, therefore, very important to control the crystallinity for assuring the stable and reliable operation of thin film devices using the electroplated copper interconnections.