Decomposition of nitrous oxide (N2O) was examined by using CaO catalysts having various surface areas, and structure sensitivity in the title reaction was discussed. The reaction rate of N2O rapidly decreased with time-on-stream and then became almost constant. Temperature programmed desorption (TPD) after the reaction showed that produced oxygen is irreversibly adsorbed on CaO surface as a poisoning molecule. Photoluminescence spectra suggested that highly unsaturated sites were readily poisoned with adsorbed oxygen. The strong dependence of the initial activity and steady state activity on CaO surface area was observed, i.e., the reaction rate per surface area in the steady state increased linearly with the surface area, but that at the initial state depended more strongly on the surface area. Thus, it was found that N2O decomposition over CaO is a structure-sensitive reaction demanding coordinately unsaturated sites. The highly unsaturated sites are extremely active for this reaction, but readily poisoned with oxygen. On the other hand, moderately and poorly unsaturated sites are responsible for the continuous catalytic activity of N2O decomposition. The structure sensitive dependence was rationalized by assuming the number of unsaturated sites having different coordination number, such as plane, edge, and corner sites, on geometric model of CaO crystallites.