Study on the effect of load rates and critical slips in the aggregation of induction motors in transient stability simulation shows that the two aspects have great influence on the accuracy of aggregation. An improved weighting-mean aggregation method for aggregating a group of induction motors is suggested. In this method， besides capacities of motors， load rates and critical slips are also taken into account. Simulation results show that the aggregated induction motor obtained by the proposed method can simulate the original group of induction motors very well in transient stability analysis. Simulation results also show that， in case of aggregating a large number of induction motors in a distribution system considering their random characters in load rates， the aggregated parameters of the equivalent induction motor obtained by the so-called statistic synthesis based method may be acceptable usually.