Abstract: A nested transaction model is one of the most important parts of the theories and applications for modern databases and distributed object systems The synchronization between parent transaction and child transaction is the basic function of the model It is known that synchronization towards single child transaction, which is commonly used in other related works, may conflict with the implicit constraints of serializability and make the schedules inoperable After analyzing the reasons of implicit constraints, well formed nested transaction is defined to avoid the inoperability described above Finally, an algorithm is given, which can control the implicit constraints of well formed nested transaction and keep the serializability of schedules