In this paper we propose to use a Quality Response Index (QRI) to evaluate the transient stability response of electric machinery under fault conditions. This index is used to calculate a control constant for the improvement of the transient response of a machine-infinite bus system which results in an optimization problem formulation. This optimization problem is solved using genetic algorithms to obtain the best value of a control proportionality-constant parameter of the Static Synchronous Compensator (STATCOM). Results of a case study considering several conditions (operation points, faults, and clearing times) are included.