Two-phase flow patterns of CO2 in a narrow rectangular channel or a small diameter tube are very important in understanding heat transfer characteristics and in developing an appropriate heat transfer correlation. Geometric effects on two-phase flow patterns of CO2 are very significant in many respects. In the present study, the flow boiling process of CO2 in a horizontal narrow rectangular channel, having a width of 16 mm and a height of 2 mm, is visualized at various test conditions, and then the effects of mass flux are analyzed. Based on the test results, a flow regime map is developed and then compared with the existing maps and flow regime transition criteria that are selected by considering physical phenomena occurred during the flow regime transition processes of CO2. In addition, the flow transition criteria for bubbly-slug, slug-annular, and bubbly-annular flow in a horizontal narrow channel are proposed by comparing the test data with existing transition criteria. (C) 2004 Elsevier Ltd. All rights reserved.