A heat exchanger collects exhaust heat of high temperature so as to warm up an engine and to heat a cabin and a battery and reduces noise of an exhaust gas. The heat exchanger may include a case provided with an exhaust inlet at one side and an exhaust outlet at the other side, an exhaust gas flowing in through the exhaust inlet and flowing out through the exhaust outlet, a coolant pathway formed in the case and fluidly communicating with a coolant inlet and a coolant outlet so as to pass coolant, an exhaust pathway formed in a pipe shape, disposed close to the coolant pathway in the case, and provided with one end connected to the exhaust inlet and the other end connected to the exhaust outlet so as to pass the exhaust gas, a bypass pathway formed in a pipe shape, disposed at a middle portion of the exhaust pathway along a length direction thereof, provided with one end selectively connected to the exhaust inlet through a valve and the other end always connected to the exhaust outlet so as to pass the exhaust gas selectively, and a connecting pipe radially extending from an exterior circumference of the bypass pathway to the exhaust pathway so as to send a portion of the exhaust gas passing through the bypass pathway to the exhaust pathway.