A heater unit comprises a casing through which air flows in one direction from an inlet to outlets. A heater core is disposed within the casing for heating air passing therethrough. An air mix door is also disposed within the casing and has a pivotal end about which it is swingable for controlling the amount of air heated by the heater core and the amount of unheated air bypassing the same. In the above arrangement, air is difficult to pass through the heater core at a place adjacently behind the pivotal end of the air mix door whith respect to the direction of air flow. A tank portion of the heater core is disposed in the place adjacently behind the pivotal end of the air mix door with respect to the direction of air flow.