In this paper, we investigate a charging mechanism for the system interworking between Wireless Local Area Networks(LANs) and Cellular Networks. Because the charging mechanisms of the two networks are different, a unified charging mechanism is required to correlate the charging informations of the two networks in the system interworking. Therefore, we propose a unified charging mechanism to collect charging information with a combined identifier. Also, we propose a decision method to control the interval of transferring accounting information according to the charging types of users (pre-paid, off-paid, and fixed-rate) and show that the proposed decision method improves the granularity and the communication efficiency of charging informations.