Abstract: In a waste heat recovery system for a liquid-cooled internal combustion engine, including a water jacket of a liquid-cooled internal combustion engine in which a portion of the heat generating in the engine is absorbed by an engine cooling liquid, a first heat exchanger is provided for heat absorption from the exhaust gas in which the exhaust gas heat is absorbed thereby and a second heat exchanger is provided for heat recovery in which the waste heat is recovered from the engine cooling liquid carrying the above-mentioned heat. A second circuit for waste heat recovery from the engine includes the first heat exchanger and the second heat exchanger, both being connected to the water jacket in series. The second circuit is adapted to return the engine cooling liquid within the water jacket to the water jacket through the first heat exchanger and the second heat exchanger by a forced circulation force or a natural circulation force.