A heat exchanger for a vehicle includes a heat radiating portion provided with first and second connecting lines formed by alternately stacking a plurality of plates and first and second operating fluids heat-exchanging with each other while passing through the first and second connecting lines; a bifurcating portion connecting an inflow hole for flowing one of the operating fluids with an exhaust hole for exhausting the one operating fluid, and adapted for the one operating fluid to bypass the heat radiating portion according to a temperature of the one operating fluid; and a valve unit mounted at the inflow hole forming the bifurcating portion and adapted to flow the operating fluid selectively to the heat radiating portion or the bifurcating portion according to a temperature of the one operating fluid flowing into the inflow hole.