A dynamic searching method of provisioning instance identifiers (PRIDs) is disclosed, which finds available PRIDs used by a network policy in data tables when adding the network policy to a policy database. A last data table without PRID assignment is first founded and then the number of used PRIDs in the last data table is retrieved to determine whether the last data table has an available PRID. If so, a field value for the available PRID is determined by the greatest or smallest field value used by the used PRIDs, or the available PRID is determined and assigned by dynamically comparing a count from a counter with the first and last PRIDs. After all data tables are successfully assigned with corresponding available PRIDs, the network policy is processed completely.