Co3Mo3N catalyst was found to be more active than the doubly-promoted iron catalyst for ammonia synthesis. The precursor CoMoO(4)(.)nH(2)O was heated under a 160 ml min(-1) flow of NH3 gas at 5.0 Kmin(-1) to 973 K and kept at 973 K for 6 h. The sample was quenched during the nitridation process and we examined how the Co3Mo3N phase was formed. This Co3Mo3N phase was formed at the last stage of NH3 treatment when the sample was kept at 973 K for 6 h. However, Mo2N and Co phases still remained at this stage. The formation of Co3Mo3N bimetallic nitride phase became much slower after caesium addition to CoMoO(4)(.)nH(2)O. The ammonia activity of Co3Mo3N catalyst was found to be promoted by the treatment with N-2 + 3H(2) reactant gas at 873 or 973 K. Co3Mo3N catalysts once treated with diluted oxygen were treated with the reactant gas (N-2 + 3H(2), 60 ml min(-1)) at 873 or 973 K for various times. The rates were increased by this treatment, although the surface areas were decreased. By keeping the sample at 873 K for 12 h or at 973 K for 3 h in the reactant gas, the activities of Co3Mo3N catalysts reached the maximum. XRD, elemental analyses, and TEM measurements revealed that the complete Co3Mo3N phase was achieved by the reactant gas treatment. The rate at 673 K under 3.1 MPa with a flow rate of 60 ml min(-1) of N-2 + 3H(2) over 2 mol% Cs-promoted catalyst reached even 40% of the equilibrium yield.