The Methanol to olefins (MTO) and ethanol to olefins (ETO) reactions were compared under the similar operation conditions, and it was proved that both follow the different reaction mechanism over H-ZSM-5 catalysts. In MTO reaction, dimethyl ether (DME) acts as the initial intermediate, which then follows two different reaction pathways depending on the acidity of H-ZSM-5 catalysts, namely, the dehydration of DME into ethene over higher acidity of H-ZSM-5 catalysts and the merization of DME through hydrogen bonds into (DME)n (n = 2-4) complexes over lower acidity of H-ZSM-5 catalysts. Further, over higher acidity of H-ZSM-5 catalysts, ethene converts into propene and butene, and thereafter undergoes the oligomerization-craking-aromatization route to form other olefins, paraffins and aromatics. But over lower acidity of H-ZSM-5 catalysts, (DME)n complexes tend to transform into high even number of olefins that are easily cracked into small olefins. In the case of ETO reaction, ethene acts as the main reaction intermediate, which then transforms into propene and butene, and further proceeds through the oligomerization-craking-aromatization route to form other olefins, paraffins and aromatics products over H-ZSM-5 catalysts.