Considering the problem that the nonlinear tracking-differentiators(NTD)cannot track all the in-put signals without error, the steady-state performance of the nonlinear tracking-differentiators in three different forms was discussed.The applicable scope of NTD was determined through analysis the characteristics of re-strain function and steady-state error in typical input, and the deductive results were verified in simulation.The results show that the nonlinear tracking-differentiators are first-order indiscrimination systems which only can track the step signal without error.