Method for selecting optimal radio access technology and communication apparatuses utilizing the same

A communication apparatus is provided. A first subscriber identity card camps on a first cell, which is associated with a first radio access technology (RAT) and belongs to a first wireless network, via a radio transceiver module. A second subscriber identity card camps on a second cell, which is associated with a second RAT and belongs to a second wireless network, via the radio transceiver module. A processor constructs a first packet switch connection with the first wireless network via the first subscriber identity card to perform data transfer in the first wireless network, measures a first data transfer throughput of the first wireless network when performing the data transfer via the first subscriber identity card, and determines to perform the data transfer in the second wireless network via the second subscriber identity card when a theoretical data transfer throughput corresponding to the second RAT is higher than the first data transfer throughput.