Abstract: Commercial HZSM-5 molecular sieves with SiO<sub<2</sub</Al<sub<2</sub<O<sub<3</sub< ratio of 200 were modified using different concentrations of sodium hydroxide solution.Their crystal structures, pore structures, morphologies and acidity were characterized by XRD, SEM, NH<sub<3</sub<-TPD and N<sub<2</sub< adsorption and desorption measurements.The catalytic performances of the modified HZSM-5 molecular sieves for the catalytic cracking of waste tires were investigated. The results show that pore structure and acidity of modified HZSM-5 can be adjusted by changing the concentration of sodium hydroxide solution. With the increase of the concentration of sodium hydroxide solution, the average pore size of HZSM-5 increases gradually, while the mesoporous volume and the amount of acid increases at first and then decreases.The HZSM-5 zeolite modified with 0.4 mol/L of sodium hydroxide solution shows the most mesoporous pore volume and acid content. When it was used to catalyze the cracking of waste tires, the selectivity of light olefins is high up to 32.39%, being much higher than that of commercial HZSM-5 zeolites.