Small Message Criterion (SMC) can measure the capability of the covert channel on transmitting small messages and is a necessary complement to the capacity criterion. However, SMC’s present definition hasdeficiencies. The acquirement of message length proved to be hard in the common information system. Mitigatingmechanism can not simultaneously satisfy the two restrictions of message transfer time and fidelity. The criteriondoes not cover information of message’s sensitivity. At first, the value function of message is introduced torepresent the danger of small message transmission. Based on the value function, a new definition of SMC ispresented where the threat tolerance standard of system is represented by a threshold of message value. The valuefunction also takes message’s sensitivity into account. A mechanism for secure real-time database scenario, whichchannel. Theoretical analysis and experimental results show that with the proposed new SMC, the secure system canperform comprehensive measurement and adjustable mitigation to the threat of covert channel.combines SMC with the channel capacity, is presented to measure and mitigate the threat of transaction covert