Increasing attention has been focused on carbon dioxide (CO2) heat Pump system where the temperature level is rather low, while the operating pressure is rather high. in this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO2 at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation. (C) 2009 Elsevier Ltd. All rights reserved.