Abstract: The deactivation of catalyst in benzene alkylation with propene over H β zeolites under different conditions was investigated. The amounts and properties of coke deposited on the spent catalysts under various reaction conditions were determined. With the change of reaction temperature from 270℃to 320℃at 5.7MPa, the reacting mixture may either lie in a phase of liquid (L) or supercritical fluid (SCF), or endure a phase variance from SCF to L, which results in different catalyst lifetimes, coke amounts and coke properties. SCF with the liquidlike densities and enhanced transport properties is able to extract coke precursors in situ, thereby to prolong the catalyst life. Under the condition of 300℃and 5.7MPa, the reacting mixture was in a state of SCF or L near its critical point during the entire reaction process, and therefore the catalyst exhibits the longest life time and slowest coke deposition rate. The C/H mol ratio of the spent catalyst is related to the reaction temperature, while the burningup temperature of coke deposited in the spent catalyst increases with the prolonging of the catalyst lifetime.