Bubbles of various Sizes are produced at nucleation sites on a heated surface in subcooled flow boiling. However, although many correlations have been developed so far for the mean bubble size, systematic information is scarce for the bubble size distribution. In the present work, bubble nucleation process in water subcooled flow boiling at low pressure was observed in detail using an ITO-deposited transparent glass plate as the heated surface. It was found that bubbles of different sizes are produced at each nucleation site and moreover the mean bubble size differs considerably between different sites. It was hence considered that the reliability of subcooled flow boiling analysis codes can be improved if the effect of the bubble size distribution is included. In view of this, quantitative investigation was done for the bubble size distribution. It was shown that under the experimental conditions tested in this work, the distribution of the maximum size of individual bubbles produced at nucleation sites are fitted well with the gamma distribution. The dependences of the measured maximum bubble size distribution on important dimensionless numbers were explored to develop new correlations for the bubble size distribution. (C) 2017 Elsevier Ltd. All rights reserved.