Bubbly flow or foam flow could prevail in the subcooled flow boiling of seawater in an annulus. Compared with the bubbly flow of de-ionized water in the subcooled flow boiling, the bubble size of seawater is smaller. Further, bubbly flow with small bubbles or foam flow in a subcooled flow boiling of seawater would be affected by different heat fluxes, mass fluxes and inlet subcooling. The change in the boiling behaviors due to different conditions can be reflected in terms of variations of bubble characteristics. However, details of the characteristics of bubbles in seawater generated by the two-phase subcooled flow boiling are still limited. In the present study, bubble characteristics are measured and detailed by an image analytic method in the software Image J. The subcooled flow boiling experiments with different heat fluxes are conducted in a vertical upward annulus with artificial seawater at the inlet temperature of 90°C and mass flux of 1200 kg/m2s. The two-phase flow patterns are recorded by the high-speed camera in 6000 frame/s. To conduct measurements of bubble characteristics using Image J, images are selected as one set of measurement from the videos recording under different heat fluxes. After preprocessing of images, measurements of bubbles and fitting procedures with ellipses are conducted. Averaged values of descriptors of bubbles are obtained, including areas, perimeters, lengths of major axes, lengths of minor axes, circularities, angles, solidities, and aspect ratios, etc. It provides a possible way to investigate the details of bubbly flow or foam flow in the subcooled flow boiling of seawater.