The emerging trend of deregulating power industries requires the operator to keep the transmission network congestion free when the generation companies compete to sell electricity to the distributors or wholesale purchasers. This requires the determination of various power transfer bottlenecks in the existing network. For this purpose the real-time predetermination of an index termed "Available Transfer Capability (ATC)" is required. Transfer capability of a power system also indicates how much inter area power transfer can be increased without system security violations. In case of large power systems such as that of Bangladesh, the vital information required for the planning and operation of the system are provided from the transfer capability calculations. These details provide system bottlenecks to the planners and the limits of the power transfers to the system operators. Thus power transfer bottlenecks determination of Bangladesh Power System effectively means the calculation of ATC of the system.