A Monte Carlo (MC) random walk algorithm is employed to investigate the kinetics of N2O catalytic decomposition over three-dimensional fractals. The simulation results show that usual kinetic equations based on the mean field theory are not applicable to the reaction over fractals, and a different rate law containing a fractional reaction order is more suitable for describing the reaction kinetics over fractals. For the annihilation reaction involved in N2O decomposition over fractals, the effective reaction order is found to range from 2 to 1+2/ds depending on the annihilation constant, where ds is the fracton dimension.