In paper the methodology of complete coverage of unknown area by autonomous mobile robot is described. The main principles of complete coverage task using sensory data only are given. The shape of coverage area is defined. The modified "the way of the ox" algorithm is proposed to realize this task and its following stages are described. Definitions of two quality indicators of realization of complete coverage task are given. The behaviour based control methodology imitated reactions of animals, realized the robot's movement, is presented. The general structure of reactive and proactive behaviours is showed. The example structures of defined reactive and proactive beheaviours, which allow to achive given task, are described. The example structure of complex behaviour implemented in Matlab-Simulink environment is presented. Graphical results of simulations of complete coverage task are presented. Values of quality indicators D and E are given and discussed.