A heat exchanger is provided which can reduce a pressure drop of a refrigerant by replacing a portion or all of baffles of a related art heat exchanger with guides having a specific configuration. The heat exchanger may include a plurality of tubes in which a refrigerant may flow; a plurality of heat dissipation fins into which the plurality of tubes may be inserted, the plurality of heat dissipation fins allowing heat exchange between the refrigerant and a fluid; at least one header coupled to at least one side of the plurality of tubes, the at least one header forming a flow space for the refrigerant; and at least one guide provided inside of the at least one header to partition the flow space and to guide the refrigerant from the at least one header to the plurality of tubes. The at least one guide may include a support provided inside of the at least one header, the support having an opening formed therein, and a movable part movably provided to open and close the opening. The movable part may be movable by a liquid refrigerant in the refrigerant.