A server, a communication system, and a performance measurement method are provided. In the method, frequency spectrum information is determined. The frequency spectrum information includes multiple frequency spectrums supported by at least two mobile networks. A network configuration is generated according to the frequency spectrum information. The core network entity instructs a radio access network node (i.e., base station) to transmit a control message to a user equipment (UE) according to the network configuration. The frequency spectrums are scanned for a network performance measurement of the UE, and the UE is allowed to register to at least one its own subscribed mobile network of the at least two mobile networks but not allowed to register to the others of the at least two mobile networks. Accordingly, an easier way to motoring competitors' network performance is provided.