A method and apparatus of transmitting a measurement report message in a wireless communication system is provided. A mobile station searches for at least one closed subcarrier group (CSG) cell included in a whitelist of the mobile station, and transmits to a GSM/EDGE radio access network (GERAN) cell the measurement report message including a toggling discriminator, which indicates an optimal CSG cell that satisfies a reporting criterion among the at least one found CSG cell, and a physical cell identify (PCI) and frequency of the optimal CSG cell.