A heat exchanger for a vehicle includes a heat radiating portion including first and second connecting lines formed alternately by stacking a plurality of plates, and receiving first and second operating fluids therethrough that heat-exchange with each other while circulating unmixed through the respective first and second connecting lines, a bifurcating portion connecting an inflow hole for flowing one of the first and second operating fluids with an exhaust hole for exhausting 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 to selectively open one of the connecting lines by expansion and contraction of deformable material so as 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.