The assignment of transaction identifiers is one of the important factors influencing the per-formance of nested transactions. In this paper, the requirements which the transaction process poses on the transaction identifiers under the nested transaction model are analysed. Based on this, a series of efficient and practical assignment strategies are proposed, and the bit-based transaction identifier strategy is implemented in the object-oriented database system——KDOODB (KeDa object oriented database). At last, the performance evaluation results are given.