A data transmission rate adjustment system and method thereof comprise a driving module and a control module. The driving module performs a predetermined mathematical calculation based on a transmission rate request to generate a divisor. The control module then performs a division that divides a reference frequency by the divisor to generate a first transmission rate so that a data receiving terminal can receive data sent from a data sending terminal through the driving module based on the first transmission rate. The driving module monitors a transmission error rate of the data received by the data receiving terminal. If the transmission error rate exceeds a predetermined value, the divisor is automatically adjusted to generate a second transmission rate. The second transmission rate is also provided to replace the first transmission rate.