In this paper Particle Swarm Optimization (PSO) technique for obtaining a reduced order approximation of a single-input single-output system based on minimization of integral squared error (ISE) pertaining to a unit step input is proposed. Unlike the conventional method, ISE is circumvented by equality constraints after expressing it in frequency domain using Parseval’s theorem. Some more existing conventional reduction techniques are also considered and compared with the proposed method through a numerical example to show the validity of the proposed method.