This paper proposes a xed-time differentiator running in parallel with a feedback linearization-based controller, which allows quadrotors to track a given trajectory. The xed-time differentiator estimates outputs' derivatives in a prede ned conver- gence time, largely compensating for initial condition problems and solving the delay problem. Combined with a state reconstruction step, a whole observer-estimator- controller scheme for trajectory tracking problem of quadrotors can be constructed. Also, an LMI optimization-based algorithm to tune parameters of the differentiator is also developed here. The high performance of the proposed model is illustrated by simulation results.