Wireless sensor networks are being effectively applied in the forest fire detection. Building a model of sensors covering the entire area of the forest is the important problem. This paper proposes the coverage model using the new approach to decrease the number of sensors. With this approach, the sensors are classified into several groups; and those groups work in the alternative way but meet the complete coverage. Experimental results on the map of the forest in Dong Thap province shows that the model works well and meets the complete coverage.