In this paper, the collision avoidance behaviour in a recursive nested behaviour control structure for Unmanned Aerial Vehicles will be discussed. The architecture is an extension of the behaviour-based recursive control structure, which has been applied successfully to mobile robot applications. The system structure, as an abstraction of multiple cascaded control loops with feedback mechanisms, is robust against disturbances and model uncertainties. The obstacle awareness and collision avoidance are considered to be the most important issues in the field of Unmanned Aerial Vehicles (UAV), thus more attention will be paid to this topic. The collision avoidance utilizes a potential field method tailored for UAV application domain. The formulation of a repulsion forces based on radar sensor measurement is presented. The implementation of the collision avoidance system is also described.