Abstract: The dynamic equation of the low-speed reaction wheel used to stabilize a satellite's three-axis attitude is put forward on the base of Dahl friction model, andaccording to this the phenomenon that the reaction wheel's speed crossing zero disturbs satellites attitude is analyzed. Anonlinear observer is used to estimate thefriction torque in order to compensate it. Simulation results prove that this methodcan compensate the reaction wheel low-speed friction, and improve attitude pointingaccuracy and stability.