A heat exchanger for an air conditioner including a compressor, the heat exchanger comprising: a first header pipe to have a refrigerant, compressed by the compressor, to flow therein in the air cooling operation; a first heat exchange unit coupled to the first header pipe to receive the refrigerant flowing in the first header pipe and to thermally exchange the refrigerant with air in the air cooling operation; a bypass pipe to have the refrigerant, thermally exchanged in the first heat exchange unit, to flow therein in the air cooling operation; a bypass valve disposed in the bypass pipe, wherein the bypass valve is opened in the air cooling operation such that the refrigerant flows through the bypass pipe. a second header pipe to have the refrigerant, passed through the bypass pipe, to flow therein in the air cooling operation; wherein the first header pipe is coupled to the second header pipe, a check valve disposed in the first header pipe to prevent the refrigerant from flowing from the first header pipe to the second header pipe in the air cooling operation; and a second heat exchange unit coupled to the second header pipe to receive the refrigerant flowing in the second header pipe and to thermally exchange the refrigerant with air in the air cooling operation; characterized in that the heat exchanger further comprising: a hot gas pipe configured to couple the first header pipe and the second header pipe; and a hot gas valve disposed in the hot gas pipe, wherein, in a defrosting operation, the bypass valve is closed and the hot gas valve is opened such that a part of the refrigerant which is compressed by the compressor and then flows through the first header pipe is passed through the first heat exchange unit, and remaining part of the refrigerant is passed through the second heat exchange unit via the second header pipe without being passed through the bypass pipe in the defrosting operation. a first header pipe to have a refrigerant, compressed by the compressor, to flow therein in the air cooling operation; a first heat exchange unit coupled to the first header pipe to receive the refrigerant flowing in the first header pipe and to thermally exchange the refrigerant with air in the air cooling operation; a bypass pipe to have the refrigerant, thermally exchanged in the first heat exchange unit, to flow therein in the air cooling operation; a bypass valve disposed in the bypass pipe, wherein the bypass valve is opened in the air cooling operation such that the refrigerant flows through the bypass pipe. a second header pipe to have the refrigerant, passed through the bypass pipe, to flow therein in the air cooling operation; wherein the first header pipe is coupled to the second header pipe, a check valve disposed in the first header pipe to prevent the refrigerant from flowing from the first header pipe to the second header pipe in the air cooling operation; and a second heat exchange unit coupled to the second header pipe to receive the refrigerant flowing in the second header pipe and to thermally exchange the refrigerant with air in the air cooling operation; characterized in that the heat exchanger further comprising: a hot gas pipe configured to couple the first header pipe and the second header pipe; and a hot gas valve disposed in the hot gas pipe, a hot gas pipe configured to couple the first header pipe and the second header pipe; and a hot gas valve disposed in the hot gas pipe, wherein, in a defrosting operation, the bypass valve is closed and the hot gas valve is opened such that a part of the refrigerant which is compressed by the compressor and then flows through the first header pipe is passed through the first heat exchange unit, and remaining part of the refrigerant is passed through the second heat exchange unit via the second header pipe without being passed through the bypass pipe in the defrosting operation.