The Available Transfer Capability (ATC) is defined as the measure of the transfer capability remaining in the physical transmission network for further commercial activity above already committed uses. Available Transfer Capability (ATC) calculation is a complicated task, which involves the determination I of total transfer capability (TTC), transmission reliability margin (TRM) and capability benefit margin (CBM). As the electrical power industry is restructured and the electrical power exchange is updated per hour, it is important to accurately and rapidly quantify the available transfer capability (ATC) of the transmission system. In ATC calculation,. the existing CPF method is accurate but it has long calculation time. On the contrary, the method using PTDF is fast but it has relatively a considerable error. This paper proposed QFA method, which can reduce calculation time comparing with CPF method and has few errors in ATC calculation. It proved that the method can calculate ATC more fast and accurately in case study using IEEE 24 bus RTS.