Optimum conditions of annealing atmosphere and temperature for the reduction of Mn content from the Cu-Mn alloy layer in Cu-Mn self-forming barrier process were investigated. Mn was selectively oxidized at the surface by annealing in Ar gas containing an impurity level of O2 (<0.01ppm). Resistivity of the film was decreased to 2.0 μΩcm after annealing. On the other hand, internal oxidation of Cu-Mn alloy was observed with no external protective surface oxide layer in Ar containing more than 10 ppm of O2. An optimum oxygen concentration is found to be in between 0.01 and 10 ppm in 1atm of Ar gas at 350 °C.