Abstract: Several molecular sieves with various pore structures were synthesized. The pore structure and acidity of these molecular sieves were characterized by BET and NH3-TPD, their catalytic performance in xylene isomerization was investigated as well. Test results indicate that both the pore structure and acidity of catalyst have significant effect on xylene isomerization. The types of reactions happened are mostly depended on the pore structure, overlarge pores are favored for transalkylation reactions take place to form heavy aromatics, which leads to increase the loss of C8 aromatics; on the other hand, extreme small pores could also increase the loss of C8 aromatics by cracking or dealkylation reactions. The catalyst activity and selectivity are closely related with the catalyst acidity, unsuitable acidity leads to increase the loss of C8 aromatics as well. Therefore, catalyst having good reaction performance in xylene isomerization should have appropriate pore structure and acidity simultaneously.