We have proposed a novel bonding process using composite Ag nanoparticles composed of Ag metallo-organic nanoparticles and Ag2CO3 for an application to the assembly of electronic devices. In this research, the sintering mechanisms of the composite Ag nanoparticles are discussed based on the results of the observation of the sintering behaviors and the investigation of the thermal characteristics. Moreover, Cu specimens were bonded using the composite Ag nanoparticles for measuring the bonding strengths. Based on the results, the effects of the Ag2CO3 contents in the composite Ag nanoparticles and the bonding conditions on the bondability were evaluated. As a result, it was found that the composite Ag nanoparticles were sintered rapidly because of the interaction between the Ag metallo-organic nanoparticles and Ag2CO3. Thereby, the bondability was improved by optimizing the contents of Ag2CO3 in the composite Ag nanoparticles.