This paper proposed an method for road friction coefficient estimation based on wheel speed and braking torque of front and rear wheels in braking condition. The braking dynamic model of two-wheeled vehicle was built in Matlab/Simulink considering load transfer of front and rear axles. Sliding mode control method was used to build the ideal braking torque controller, which control objective is to control the actual wheel slip ratio of front and rear wheels tracking the ideal wheel slip ratio. In order to eliminate chattering problem of the sliding mode controller, integral switching surface was used to design the sliding mode surface. Road friction coefficient can be observed by second order linear extended state based on wheel speed and braking torque of front and rear wheels. The simulation results show that the proposed method can estimate road friction coefficient exactly with strong robustness in different road conditions, which can resist external disturbance.