To fully exploit the potential capacity of EHV transmission lines, an integrated rating approach which combines the static temperature raising method and dynamic monitoring method is proposed which can keep secure operation without changing existing transmission line facilities. The Morgan ampacity formula is revised, and the security of dynamic rating is verified by calculating the sag of transmission line. A dynamic rating tool is developed based on Visual C++. The impact of dynamic rating method upon line ampacity is illustrated by numerical examples in different scenarios. The results demonstrate that dynamic rating of power transmission lines can usually provide a significant increase of transmission capacity compared to the static rating method, and the effect is more obvious when combining the two methods.