A data transmission coordinating method is used between a central processing unit and a bridge chip of a computer system. By entering the computer system into a coordinating state, the data transmission coordinating method is executed. The bridge chip and the CPU are informed of maximum bit numbers of each other for data transmission therebetween via the front side bus. Then, a commonly operable maximum bit number for data transmission between the CPU and the bridge chip can be coordinated according to the first and second maximum bit numbers. Once the commonly operable maximum bit number is determined, the CPU is reset to operate with the commonly operable maximum bit number. The maximum bit numbers are those of bus transmission width or bus transmission speed.