A heat exchanger for a vehicle may include a heat radiating portion provided with first and second connecting lines formed alternately by stacking plates, and receiving first and second operating fluids into 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, and the bifurcating portion is adapted for the one operating fluid to bypass the heat radiating portion according to a temperature of the one operating fluid. The heat exchanger further includes a valve unit mounted corresponding to the inflow hole and adapted to flow the one operating fluid selectively to the heat radiating portion or the bifurcating portion according to the temperature of the one operating fluid flowing in the inflow hole.