This paper addresses the problem of quadrotor pitch/roll estimation when only IMU sensors are available. A Smooth Sliding Mode (SSM) algorithm is firstly considered to provide reliable estimation of pitch/roll angles under a smoothness disturbance assumption. The performance is compared to use a recently developed high-order sliding mode (HOSM) differentiator. In addition, the observer performance is discussed for the case where measurements are corrupted by bias and noise. Simulation results obtained on the nonlinear model of quadrotor illustrate the benefits of the proposed techniques