Abstract: The effects of SiO₂/Al₂O₃ ratio of HZSM-5 on the tar quality and coke formation during catalytic upgrading of coal pyrolysis volatiles were investigated. The results showed that the carbon deposition amount decreased with the increase of the SiO₂/Al₂O₃ ratio of HZSM-5 due to the decrease of strong and weak acid amount. When the SiO₂/Al₂O₃ ratio increased from 23 to 310, the carbon deposition amount decreased from 120.1 mg/g-catalyst to 23.9 mg/g-catalyst. Moreover, the higher ratio of strong and weak acid amount led to lighter fraction in tar. With the decrease of SiO₂/Al₂O₃ ratio, the acid strength was enhanced so that the aromatics content in the tars decreased, indicating that the stronger acidic sites promoted the dehydrogenation, cyclization and aromatization reactions of volatiles, thus leading to higher yield of aromatics during catalytic coal pyrolysis.