Abstract: When reaction wheels rotate at nearly zero speed,the mom ent of friction will vary discontinuously.This leads to the uncertainty of contr ol moment,which influences the performances of satellite attitude control. Based on a nonlinear control approach,a nonlinear control law is developed to im prove attitude control performances.The analyses show the control law is convinc ing for zero-speed wheel.Finally,a simulation example is provided.Compared with traditional linear control law,the nonlinear control law is fully effective in d ealing with the uncertainties of wheels' control output. Moreover,the nonlinear control law is also quite simple.It occupies few memory c apacity compared with the linear control law and consumes a little CPU time.As a result,it is possible that the nonlinear control law is used directly on board. Subject Term Attitude control Reaction wheel Nonlinear control S atellite