A heat exchanger for a vehicle is disclosed. The heat exchanger includes a heat radiating portion provided with first and second connecting lines formed alternately by stacking a plurality of plates, and receiving first and second operating fluids respectively into the first and second connecting lines. The first and second operating fluids exchange heat with each other during passing through the first and second connecting lines. The heat exchanger also includes a bifurcating portion connecting an inflow hole for flowing one operating fluid of the first and second operating fluids with an exhaust hole for exhausting the one operating fluid, adapted for the one operating fluid to bypass the heat radiating portion according to a temperature of the one operating fluid, and mounted at an exterior of the heat radiating portion.