To clarify the N-2 formation mechanism during the C-N2O reaction, the detailed nitrogen mass balance was established. It was also attempted to reveal the role of surface nitrogen species in this reaction by using isotope gases. In contrast to the case of the C-NO reaction, the accumulation of nitrogen on the carbon surface was very small during this reaction. Despite this accumulation, the trapped surface nitrogen species hardly contribute to the N-2 formation process. The main reaction path for the N-2 formation is the attack of N2O to carbon free site, forming N-2 with no N=N splitting and leaving oxygen on the carbon surface.