A ligand-free Heck reaction catalyzed by in situ-generated palladium nanoparticles in PEG-400 has been developed. This catalytic system is a simple and active protocol for the Heck reaction of aryl halides under mild conditions. Comparative experiments demonstrated that the Heck reaction catalyzed by the palladium nanoparticles in situ-generated under the Heck reaction conditions was carried out much quicker than that by the in ex situ-generated ones.