Cu nanoparticles are promising interconnection material due to low cost and superior conductivity while they readily oxidize and need special processing and storing conditions. To solve these problems, a specific in situ reduction-sintering of CuO nanoparticles was developed and oxide free Cu submicron particles suitable for sintering were fabricated. The surfaces of the Cu submicron particles show no obvious oxide structure even after air sintering at 220 degrees C for 15 min and Cu-Cu joints with high shear strength of 22 MPa were produced. The oxide forms during longer bonding durations had a morphological evolution from stripe into grain and developed different layer structures on Cu particle surface. The micro-fracture mechanism of sintered Cu particles was analyzed and Cu particles were found to deform plastically while the surface oxide show obvious brittle fracture. Variation in shear strength with bonding time was analyzed and simulated based on contact area theory and the degradation of shear strength was correlated to the oxide formation. The proposed method produces similar shear strengths to Cu nanoparticle sintered joints but without the need for pressurized sintering or protective gas atmospheres. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.