The cationic polymerizations of isobutylene (IB) coinitiated by AlCl3 were carried out in solvent mixture of n-hexane/methylene dichloride (n-hex/CH2Cl2) of 60/40 V/V in the presence of ethyl benzoate (EB) at various temperatures range from -80°C to -30°C. The effects of EB concentration ([EB]) and polymerization temperature on monomer conversion, weight-average molecular weight (Mw) and molecular weight distribution (MWD, Mw/Mn) of polyisobutylene (PIB) products were investigated. The rate of polymerization decreased while Mw of PIB products increased with increasing [EB]. The polymers with high molecular weight could be prepared in the presence of a suitable amount of EB. Significantly, the polymers with high Mw of 80.2×104 and 65.4 ×104 could be produced at -80°C and -70°C at [EB] = 0.24 ×10-3 mol/L respectively, which were much higher than that (Mw = 57.9 ×104) of PIB prepared at -100°C in the absence of EB. A simple but effective method for preparing the high molecular weight polyisobutylenes was developed in this work. It has been also found that the activation energy for propagation (Ep) depended on the polymerization temperature range in the presence of EB. An obvious inflection of the linear plots of lnXn versus 1/Tp occurred at the temperature range from -60°C to -50°C at four different concentrations of EB from 0.19 ×10-3 mol/L to 0.33 ×10-3 mol/L, and thus the inflection temperature (Tinf) was in the range of -60°C to -50°C. When [EB] was in the range of 0.24 ×10-3 mol/L to 0.33×10-3 mol/L, Ep was determined to be around -12 kJ/mol when the polymerization was carried out at temperatures from-80°C to Tinf and to be around-28 kJ/mol at temperatures from Tinf to-15°C respectively.