An argument has lasted for several decades in studying the mechanism of Pd nanoparticles-catazyzed Suzuki-Miyaura coupling reactions, heterogeneous or homogeneous catalysis. By using in-situ surface-enhanced Raman spectroscopy (SERS) with high specificity and selectivity, the real catalytically-active sites of Pd nanoparticles for Suzuki coupling reactions are investigated and heterogeneous catalysis is concluded. The research provides new opportunity in mechanistic study of interfacial reactions by using SERS which will benefit the rational design of high-performance Pd catalysts.