In this paper, a nonlinear complimentary filter (x-estimator) is presented to estimate the attitude of a UAV (unmanned aerial vehicle). The measurements are taken from a low-cost SMU (inertial measurement unit) which consists of 3-axis accelerometers and 3-axis gyroscopes. The gyro bias are estimated online. A second nonlinear complimentary filter (z-estimator) is also designed, it combines 3-axis gyroscope readings with 3-axis magnetometer measurements. From the proposed estimators, the full rotation matrix R will be retrieved. Both estimators use the fact that the orientation matrix, evolving on SO(3), is not locally parameterized and thus could be used to describe any kind of 3D motion. Convergence of the two observers is theoretically proved and simulations as well as experiments are conducted on a real platform in hovering flight conditions.