Aerial manipulation systems become very attractive for a wide range of applications due to their unique features. However, control of such system is quite challenging due to its high nonlinearities, couplings, and external disturbances. In this paper, a Disturbance Observer (DOb)-based linearization of a quadrotor manipulation system is utilized. The DOb estimates the disturbances and nonlinearities, then compensates them such that one can treat the control problem based on a simple linear control algorithm. However, the current developed DOb schemes in the literature are based on the precise measurement of the acceleration or the estimation of the velocity. Unlike, these methods, we propose a modified DOb, which is based on both the measured linear accelerations and angular velocities that can be obtained directly from the onboard Inertial Measurement Unit (IMU) and encoders. With this technique, the estimation of nonlinearities and disturbances are carried out without the need of estimation acceleration or velocity, and it is model free. The Effectiveness of the proposed technique is verified via numerical simulations.