There is provided a communication system including a master device and a slave device. The master device includes a first interface port and a second interface port. When the master device runs a read operation, the first interface port is used to output a serial clock to the slave device. The slave device includes a clock generator used to generate a delayed serial clock having the same phase as read data according to the serial clock. The second interface port is used to receive the delayed serial clock and the read data to eliminate a time delay in a data transaction.