Abstract: The importance of two-phase instabilities has attracted substantial efforts to study the physical phenomena governing such instabilities. The paper is to present a numerical model derived from the basic conservation equations for analyzing flow-induced instabilities in once-through steam generator(OTSG) with annular tubes. The assumption of homogenous equilibrium two-phase flow was adopted in the model, and the thermal capacity of the tube wall was included. The model was used to study the effects of system pressure, flow flux, inlet subcooling, inlet/outlet throttling and the annulus diameter ratio of d_o/d_i on system behavior.