Disclosed is a context synchronization method in a mobile communication system dealing with a packet transmission/reception. If two peers of a protocol have the same information and their data are not frequently updated, the invention makes the QUERY about the STATUS or CONTEXT of the relative side to acquire the previous information of the relative side, thereby enabling to cope with the situation more flexibly. In a protocol of a radio system having a client and a server communicating with each other, the invention includes the steps of performing idle and normal procedures in the client, performing a recovery procedure if an error occurs in performing the idle and normal procedures, and performing a re-attach procedure and an activation procedure according to the error and an error occurring point. Also, the invention includes the steps of having a mobile station (MS) to select a cell fitting for a corresponding network to register the mobile station to the network, performing general idle and normal procedures, and performing a recovery procedure if a failure occurs.