A computer system has an adjustable data transmission rate between a CPU and a core logic chip thereof. In the computer system, the CPU has a power state adjustable in response to a power management control signal issued by the core logic chip. For adjusting data transmission rate between the CPU and the core logic chip, a change of an asserted time of the power management control signal from a first time period to a second time period is first determined to obtain an index value. The data transmission rate is increased or decreased according to the index value.