STATCOM controlled power system shows highly non-linear characteristics. In such a structure, linear controllers are easy to apply but difficult to tune. Fine tuning of controller parameters is necessary to meet transient performance requirements. In this paper, control system parameters of a STATCOM which is constructed from two six-pulse voltage source based converters are optimized by Simplex algorithm which is a non-linear and non-gradient method. System objective function to be minimized is based on a performance index and expressed in terms of measured and desired values of system variables. After optimization, performance of STATCOM control system consisting of two PI controllers and two leag/lag compensators has been tested and verified through a number of simulation studies investigating step-response performance. Precise and quick responses have been observed with optimized parameters.