t-Butylation of toluene with t-butylalcohol (t-BuOH) as alkylating agent was conducted under liquid phase reaction conditions over Al-MCM-41 with different Si/Al ratios for highly selective synthesis of 4-t-butyltoluene. For the reactions, the influences of various reaction parameters such as reaction temperature, time and t-BuOH to toluene ratio are discussed. With increasing the Si/Al ratios of Al-MCM-41 catalysts from 21 to 104, the conversion of toluene, and the yield and selectivity of 4-t-butyltoluene decreased because the number of Brønsted acid sites in Al-MCM-41catalysts is found to decrease almost linearly with increasing ratios of Si/Al. The conversion and selectivity is increased in Al-MCM-41(21) with each cycling of reaction. The Si/Al ratio of 21 is found to be more suitable for the t-butylation of toluene to highly selective synthesis of 4-t-butyltoluene. Thus the selectivity of 4-t-butyltoluene is higher in Al-MCM-41(21) than that in other Al-MCM-41 samples.