The reactivity and the reaction conditions of the ring-opening polymerization of -caprolactone (-CL) and -lactide (LA) initiated by aluminum ketiminate complexes have been shown differently. Herein, we account for the observation by studying the mechanisms on the basis of density functional theory (DFT) calculations. The calculations show that the ring-opening polymerization of -CL and LA are rate-determined by the benzoxide insertion and the C-O bond cleavage step, respectively. Theoretical computations suggest that the reaction temperature of -LA polymerization should be higher than that of -CL one, in agreement with the experimental data. To provide a reasonable interpretation of the experimental results and to give an insight into the catalyst design, the influence of the electronic, steric, and thermal effects on the polymerization behaviors will be also discussed in this study.