A reservoir tank includes a tank portion having a cover coupled to an upper portion thereof, and having coolants with different temperatures supplied thereto, a heat exchange reduction portion partitioning an internal space of the tank portion into a first accommodation space and a second accommodation space, and having the first accommodation space and the second accommodation space formed to be spaced from each other, and a discharge portion formed in the heat exchange reduction portion and formed to allow coolant flowing into the heat exchange reduction portion to be discharged back to the first accommodation space and the second accommodation space, respectively.