An adapter for an engine cooling system is provided. The adapter, includes a main pipe, having a first end coupled to a first degassing line connected to a high-temperature component, and a second end of coupled to a second degassing line connected to a reservoir tank. A branch pipe is branched from one side of the main pipe to be coupled to a third degassing line connected to the low-temperature component. An inner pipe is disposed at an interior circumference of the main pipe and has a diameter that is gradually reduced toward the reservoir tank.
