The influence of particle size of HZSM-5 zeolite on selective catalytic reduction of NO by acetylene (C2H2-SCR) was investigated. The zeolite with nano-particle behaved considerable higher activity than the micro-particle one for the reaction. It was revealed that the large difference in the activity for the C2H2-SCR of NO arising from the particle size of zeolite was not caused by limited intracrystalline diffusion of the reductant, but that of NO2, which was strongly supported by the adsorption results obtained over the zeolites.