The reliability of solder joints, which is affected largely by the interfacial intermetallic compounds layer, is of great importance to the performance of electronic devices. In this paper, a novel composite solder with Mo nanoparticles doped in Sn3.0Ag0.5Cu solder was prepared. Solder/Cu joints were prepared using Sn3.0Ag0.5Cu solder or Mo nanoparticles reinforced solder with Cu substrates. The effect of the nanoparticles on the growth of intermetallic compounds during isothermal aging was investigated. It was found that the addition of Mo nanoparticle markedly suppressed the increase in thickness of the interfacial intermetallic compounds layer during isothermal aging. The atomic diffusion coefficients during isothermal aging in Sn3.0Ag0.5Cu solder and in the novel doped solder were determined experimentally. Mo nanoparticles were found to decrease the diffusion coefficient in the composite solder. The high specific surface area of the nanoparticles makes them easily adsorbed and hinders the diffusion of Sn and Cu atoms. This is consistent with an increase in the diffusion activation energy due to the addition of Mo nanoparticles in Sn3.0Ag0.5Cu solder.