The application of microporous sintered copper (Cu) as a bonding material to replace conventional die-attach materials in power electronic devices has attracted considerable interest. Many previous studies have focused on the effect of processing parameters (temperature, time, pressure) on the microstructure evolution of sintered Cu. However, there are only a few studies with regard to the mechanical properties of sintered Cu. As the die-attach layer undergoes thermal and mechanical stress durin g its application, it is essential to investigate the micro-scale mechanical properties of sintered Cu. Fracture toughness is a measure of the resistance of a material to crack propagation under predominantly linear-elastic conditions, which is an essential para meter for predictin g fracture failure. As cracks and defects are difficult to avoid during fabrication and application processing for sintered Cu, which will defin itely cause a sign ificant effect on micromechanical properties. Thus, it is essential to reveal the effect of microstructure on fracture toughess of sintered Cu nanoparticles.