The effect of different-sized Cu6Sn5 nanoparticles on thermal behavior and interfacial reaction with Cu substrate of Sn–0.3Ag–0.7Cu solder was evaluated. The results showed that 30 nm and 70 nm Cu6Sn5 nanoparticles were successfully synthesized by using the method of different reductants, attributed to the difference of reductive ability. Under condition of the same heating parameters, 30 nm Cu6Sn5 nanoparticle owned lower sintering temperature than that of 70 nm Cu6Sn5 nanoparticle. The thermal behavior was revealed that the composite solders with the addition of different-sized Cu6Sn5 nanoparticles had slightly lower melting point and higher undercooling value, compared with original Sn–0.3Ag–0.7Cu solder alloy. The highest undercooling value occurred with 30 nm Cu6Sn5 nanoparticle addition. Moreover, the interfacial reactions between Cu substrate and Sn–0.3Ag–0.7Cu solders mixed with different-sized Cu6Sn5 nanoparticles have produced after reflowing at 250 °C and aging at 150 °C for different interval time. It was clearly found that comparing to the initial Sn–0.3Ag–0.7Cu/Cu solder joint, the thickness of interfacial intermetallic compound (IMC) layer on solder joint with adding nanoparticle was thinner and the growth rate was slower. The growth of interfacial IMC was suppressed with the addition of Cu6Sn5 nanoparticle, and 30 nm Cu6Sn5 nanoparticle had the strongest inhibition effect on the growth of IMC, as well as the growth of IMC grains.