In this paper, an experimental investigation was carried out for the characteristics of flow instability with the evolution of two-phase interface morphology. The inner diameters of test sections included 2.15 mm, 4.95 mm, as well as 6.80 mm, and the inner diameter of bypass was 12.5 mm. The deionized water was selected as the working fluid, and the characteristics of two-phase interface morphology were recorded by a high-speed camera. It was found that the corresponding flow patterns were the typical annular flow for all the test sections as the two-phase flow instability occurred. As for the test sections (D-i(n) = 6.80 mm and D-i(n) = 4.95 mm), the flow patterns gradually transferred from the bubbly flow to the annular flow with the heating flux increasing, and the flow instability finally happened as the annular flow appeared. As for the test section (D-in = 2.15 mm), the flow state remained stable before OFI (Onset of Flow Instability) with the single phase or bubbly flow. On account of the significant channel restriction and surface tension, the flow pattern rapidly evolved to the annular flow for OFI, while the flow state simultaneously transferring from the stable state to the unstable state. Finally, a prediction model of the flow instability for test sections with different inner diameters (D-in = 2.15 mm, D-in = 4.95 mm, D-in = 6.88 mm) was established combined with the dimensionless parameters of N-conf and We; the predicted results agreed well with the experimental results. (C) 2019 Elsevier Ltd. All rights reserved.