The present invention relates to a method for controlling the temperature of a cooling fluid in a cooling system (6) arranged in fluid connection with an internal combustion engine (2) of a vehicle, the cooling system comprising a cooling circuit (50) connected to a condenser (26) of a waste heat recovery system (4), a cooling pump (52) arranged to circulate a cooling fluid through the cooling circuit (50) and a radiator (54) arranged for cooling the cooling fluid, a bypass circuit (56), a first valve unit (58) and a second valve unit (60), wherein the first valve unit (58) is arranged to control the flow of cooling fluid through the radiator (54) and the bypass circuit (56) respectively, and the second valve unit (60) is arranged to control the flow of cooling fluid passing through the condenser (26). A condenser-in temperature of the cooling fluid is determined by means of a first temperature sensor (55) and an engine-in temperature of the cooling fluid is determined by means of a second temperature sensor (57). The flow of the cooling fluid is controlled by means of the first valve unit (58) and/or the second valve unit (60) based on the determined condenser-in and engine-in fluid temperatures.