The present invention relates to a method for fabricating a capacitor in a semiconductor device; and, more particularly, to a method for fabricating a capacitor capable of stably forming a nitride layer on a lower electrode and obtaining improvements on stable capacitance and leakage current characteristics. The inventive method for fabricating a capacitor includes the steps of: forming a lower electrode on a substrate; forming a nitride-based first dielectric thin layer on the lower electrode; forming a second dielectric thin layer by depositing an Al2O3 layer on the nitride-based first dielectric thin layer; forming a third dielectric thin layer on the second dielectric thin layer; and forming an upper electrode on the third dielectric thin layer.