The application of multiscale silver paste and bare copper could be a more economical way for power electronics. In this paper, bonding mechanism of die attachment on bare copper substrate using multiscale Ag paste containing nano- and micro-particles in air condition was clarified. After sintering, the oxide layer generated between sintered Ag and copper substrate was composed of Cu2O and Cu. The formation of Ag-Cu2O chemical bond and Ag-Cu metallic bond was clearly confirmed based on the lattice structure analysis at the interface between sintered Ag and oxide layer. The lattice images also indicated Ag/Cu2O chemical bonding could be even stronger than Ag/Cu metallic bonding due to the smaller mismatch of lattice constant with Ag. Besides, two copper substrates with different grain sizes were designed to compare their corresponding interfacial diffusion behavior and fracture condition. The copper substrate with smaller grain size provided more grain boundaries to accelerate Ag atoms diffusion on the surface before oxidation, further increasing the adhesive area of Ag. Then the larger area of Ag aggregation decreased the content of oxide layer thickness and voids at the interface between copper oxide and substrate, leading to the stronger interfacial bonding and higher joint strength.