Road friction coefficient real-time estimation methods is an important issue and problem in automotive active safety control system development. First a fixed feedback gain sliding mode observer of road adhesion coefficient is designed through the establishment of tire/road dynamic friction model in this article. The simulation results shows that the observer can well real-time identify the current road adhesion characteristics. And more importantly, the observer only need wheel speed signal and the braking torque(brake pressure) signal, so the system is low cost, and its adaptability is good. There is no doubt this estimation method has a good application prospect. Road friction coefficient real-time estimation methods is an important issue and problem in automotive active safety control system development. First a fixed feedback gain sliding mode observer of road adhesion coefficient is designed through the establishment of tire / road dynamic friction model in this article. And the importantly, the observer only need wheel speed signal and the braking torque (brake pressure) signal, so the system is low cost, and its adaptability there is no doubt this estimation method has a good application prospect.