Among 11 kinds of molecular sieves examined, H-ZSM-5 exhibited the highest activity for the direct conversion of ethylene to propylene. The conversion of ethylene was 58% and the selectivities to propylene and butenes were 42% and 21%, respectively, over H-ZSM-5 at 723 K. Our studies demonstrate that the conversion of ethylene increases with the degree of H+ exchange in the H-Na-ZSM-5 series of samples with different H+ exchange degrees and the Al content in the H-ZSM-5 samples with different SUM ratios. The strong Bronsted acid sites are proposed to account for the conversion of ethylene. The modification of H-ZSM-5 with phosphorus or boron could enhance the selectivity of propylene but decreased the conversion of ethylene due to the decreased acidity. In situ FT-IR studies confirm the reaction of ethylene molecules with the Bronsted acid sites associated with -Si-(OH)-Al- groups. In situ FT-IR results further suggest the occurrence of oligomerization of ethylene on the surface of H-ZSM-5. We speculate that the cracking of the oligomeric intermediates may lead to the formation of propylene.