In this article, a Modified Firefly Algorithm (MFFA) based optimal location and sizing of unified power flow controller (UPFC) to progress the dynamic stability is projected. At this point, the maximum power loss bus is acknowledged at the most auspicious location for fixing the UPFC, as the generator outage disturbs the power flow constraints like power loss, voltage and active and reactive power flow. An FFA (Firefly algorithm) is explored with the help of gravitational search algorithm (GSA). The projected algorithm progresses the loadability of power scheme with UPFC. Haphazard movement factor of firefly algorithm is enhanced by hybridizing the GSA with FFA. In outmoded firefly algorithm, the subsequent movement of firefly is hinge on the movement factor that is dogged by haphazardly so the best movement of firefly is prospect to fails by the dissemination of haphazard number. Therefore, the best location for placing and capability of UPFC can severable to identify precisely. In this article, a GSA based modified optimization algorithm is utilized to regulate the optimal haphazard movement factor of fireflies. Therefore, the optimal location and capability of UPFC is dogged competently if associated with traditional FFA. Lastly, the projected work is applied in MATLAB/Simulink and the optimal location and capacity of UPFC is scrutinized as per the variation of voltage, power loss and power balance of the network. To validate our anticipated method, we complete replications on an IEEE 30-bus power scheme. The solutions we have attained designate that installing UPFC in the location augmented by MFFA can meaningfully improve the loadability of power scheme by reducing the overloaded lines and the bus voltage edge violations. © 2018, World Scientific and Engineering Academy and Society. All rights reserved.