In this study, data classification and modeling of the flooding phenomenon were performed in a narrow channel with a gap thickness of several millimeters. To implement the conventional classification study to a narrow channel case, three key ideas for two-phase flow in a narrow channel were proposed; characteristic length, the flow path for each phase and a concept of unit-cell that can unify both narrow rectangular and annular channels including large outer diameter. Based on the ideas, 330 flooding data points for both narrow rectangular and annular channels including large outer diameter were classified into exit flooding and entrance flooding according to the geometries of the liquid inlet. In order to develop the prediction model for flooding in a narrow channel, the hyperbolicity breaking concept was introduced. The present flooding model was derived based on the hyperbolicity breaking of the two-fluid model and produced the Root Mean Square Errors (RMSE) of 30.02% in terms of the superficial gas velocity against the 330 flooding data points from narrow rectangular and annular channels with relatively small outer diameter. Moreover, the proposed model was applied to a case of narrowlarge diameter annuli with the introduction of the number of the unit-cells producing the RMSE of 26.11% against 91 flooding data points. The RMSEs of the existing models for narrow rectangular and narrow large-diameter annuli were 38.25% and 31.10%, respectively. Different from the existing models, the proposed model turned out that it is applicable to various narrow channel-types; a rectangular, an annular and an annulus with a large diameter.