The friction force is the most important factor for the design of control unit in a braking system. For a long time, many researchers have been striving to improve the accuracy in the measurements of friction coefficients [1,2]. However, there were many difficulties because the friction coefficients are affected by a number of conditions and parameters, such as normal force, temperature, characteristics of road condition, and weather. For the development of ABS of the aircraft, the HILS (Hardware-In-the-Loop-Simulation) test and dynamometer test were carried out. For the calculation of the friction coefficients, the wheel moment was measured using the load cell mounted on the housing of the wheel. The test conditions were dry and greasy, with friction coefficients of 0.7 and 0.4, respectively. In this paper, the test results of the friction coefficients were represented and the improvement method was suggested.