In a power system, transmission reliability margin (TRM) is a key factor that determines the available transmission capability (ATC) ensuring the secure operation of the transmission network during the occurrence of uncertainties. Before transmitting available power through the network, it is necessary to know the secure margin. The secure margin determines whether it’s safe for transmission or not. The exact calculation of the transmission reliability margin is quite challenging due to the random disturbances in the transmission network. This paper introduces an effective technique for determining the TRM by AC load flow, considering the available transmission capability and sensitivity of three distinct system parameters such as load, transmission line impedance, and bus voltage magnitude. Numerical results demonstrate that the proposed technique is an attractive solution for calculating the ATC, sensitivity with respect to ATC, and TRM considering the effect of system parameters. The whole process is done for the standard IEEE-6 bus system considering multi-transactions. Finally, the calculated TRM values are compared with the existing techniques for justifying the effectiveness of the proposed technique.