This paper describes an efficient reduced order model (ROM) applied in the aerodynamic analysis of bluff bodies. The proposed method, which is based on eigensystem realization algorithm (ERA), uses the impulse response of the system obtained by computational fluid dynamics (CFD) analysis to construct a ROM that can accurately predict the response of the system to any arbitrary input. In order to study the performance of the proposed technique, three different geometries including elliptical and rectangular sections as well as the deck cross section of Great Belt Bridge (GBB) were considered. The aerodynamic coefficients of the impulse responses of the three sections are used to construct the corresponding ROM for each section. Then, the aerodynamic coefficients from an arbitrary sinusoidal input obtained by CFD are compared with the predicted one using the ROM. The results presented illustrate the ability of the proposed technique to predict responses of the systems to arbitrary sinusoidal and other generic inputs, with significant savings in terms of CPU time when compared with most CFD codes. The methodology described in this paper has wide application in many offshore engineering problems where flexible structures interact with unsteady fluid flow, and should be useful in preliminary design, in design optimization, and in control algorithm development.