A method of fabricating a metal plug. On a semiconductor substrate comprising a MOS device, a dielectric layer, and a via hole penetrating though the dielectric layer, a conformal titanium layer is formed on the dielectric layer and the via hole. A low temperature annealing is formed in a nitrogen environment, so that a surface of the titanium layer is transformed into a first thin titanium nitride layer. A conformal second titanium nitride layer is formed on the first thin titanium nitride layer by using collimator sputtering. A metal layer is formed and etched back on the second titanium nitride layer to form a metal plug.