A cooling system for a heavy road vehicle provided with a hybrid propulsion system includes a mechanical propulsion system and a hydraulic propulsion system. The cooling system includes a pump and a first cooling circuit including a gearbox cooling circuit a hydraulic propulsion system cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit. The radiator is connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion system cooling circuit. The cooling system is designed such that the hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel.