Abstract: In this paper, the effect of silicon/aluminum ratio and grain size on pore structure and acidity of ZSM-5 molecular sieve was investigated by N2 adsorption-desorption and NH3-TPD methods, and the hydrogenation performance of modified ZSM-5 molecular sieves and Pd/ZSM-5 catalysts were evaluated using heptane and cyclohexane as feeds. Results indicate that increasing the ratio of silicon and aluminum will reduce the acid sites of the catalyst and relatively increase the amount of hydrogenation active center. Due to reduction of secondary cracking, the selectivity of isomerization is improved; the nano-size ZSM-5 greatly improves the activity of the catalyst. The Pd loading changes the reaction pathway and lowers dawn the alkanes activation energy, resulting in improvement of hydrogenation activity. When the Pd loading is larger than 0.4%, the hydrogenation activity of the catalyst will level off.