Recently, with the advance of communication technologies and sensors, the so-called dynamic rating is now to be realized. The dynamic rating is a method to determine accurate ratings by utilizing real-time information such as conductor temperatures, ambient temperatures and wind speed. The dynamic rating is considered to be available for the efficient use of network under the deregulated environment. In this paper, the validity of the dynamic rating from the viewpoint of ATC is discussed, and also by considering the relationship between ATC with thermal constraints and weather conditions, ATC is estimated when the dynamic rating is applied to a power system.