The behavior of condensing vapor bubbles in subcooled bulk flow is a complicated phenomenon influenced by both heat and mass transfer. To clarify the characteristics of such a thermal-nonequilibrium two-phase flow, an experimental research has been done using a boiling water loop. The movement of vapor bubbles flowing in the transparent test section was recorded by a video camera. An image-processing technique was newly introduced to measure the distributions of void fraction along the test section. The average density of two-phase fluid was also measured by using a density meter. These void fraction data plotted against inlet liquid subcooling were located midway between two curves which were predicted respectively from the Saha-Zuber subcooled boiling theory and an assumption considering the thermal-equilibrium condition. From these visual observations, two different types of bubbles were found in their condensing process during flowing in the same subcooled bulk flow, and they were largely influenced by subcooling distributed in their surrounding liquid.