Abstract: Based on force balance analysis under static condition, a bubble departure model was established to predict the bubble departure size under rolling condition by considering the additional force. The proposed model agrees well with the experimental data. By force analysis it is indicated that the buoyancy fluctuation caused by rolling motion must be considered in the model. The calculation result shows that the bubble departure diameter is the minimum when the angular displacement is zero. This tendency is more unobvious with the increase of mass flow rate. Moreover, if the experiment loop has more than 5% of the flow fluctuation, the influence of flow fluctuation on bubble departure diameter must be considered.