Investigations on the effect of the local structures on Fe-ZSM-5 zeolites by means of ESR, UV-Vis absorption and XRD measurements have shown that Fe-ZSM-5 with low Si/Al ratios includes highly dispersed Fe-oxides with tetrahedral and distorted tetrahedral coordinations, while the latter species could not be observed in Fe-ZSM-5 with high Si/Al ratios. Moreover, the photocatalytic decomposition of N2O into N-2 and O-2 was found to proceed on Fe-ZSM-5 having low Si/Al ratios with a good linearity against the UV-irradiation time and a good stoichiometry. It was also observed that the photocatalytic efficiency decreased when the Si/Al ratio was increased. These results clearly suggest that a highly dispersed distorted tetrahedrally coordinated Fe-oxide species plays a significant role in the photocatalytic decomposition of N2O into N-2 and O-2.