The CANDU steam generator consists of tube bundles where two phase flow across the bundle takes place. Among many tube vibration mechanisms, the FluidElastic Instability (FEI) is considered the most destructive one. Many studies focused on FEI of two phase flow. However, there is no analytical model until now able to predict FEI threshold velocity. This study presents a novel analytical framework to predict the fluidelastic instability threshold of two phase flow across normal square bundle. The model focuses on the bubbly flow (void fractions up to 35%). A single vibrating tube in a fixed bundle was considered. Flow around the vibrating tube is idealized as one dimensional bubbly flow in two channels. The fluid in each channel is composed of continuous phase and dispersed phase. The dispersed phase is accounted for by spherical bubbles. The motion of each individual bubble was modeled by accounting for the external forces acting on its surface. Bubble-to-bubble interaction, bubble break-up, and bubbles coalescence were taken into account. Bubble re-sponse to the pressure pulsations was integrated in the model by solving Rayleigh-Plesset equation. By tracking each bubble in the flow channels, it is possible to calculate the change in the flow density around the tube. The calculated instantaneous local density is used to solve the unsteady conservation equations to find the fluid forces on the vibrating tube. The tube is then modeled as a single degree of freedom system to predict its displacement. The feedback mechanism of the tube on the fluid was then modeled. Time domain simulations were conducted for air-water mixture and the stability threshold was predicted. Prediction of the stability threshold showed a very promising results when compared with the experimental data. A sensitivity study is performed on the model to test its limitations is presented.