This research presents experimental approaches to accumulate the data for mechanistic model in subcooled flow boiling. A number of photographic studies have been provided to investigate phenomena of bubble nucleation and condensation process for accurate prediction of void fraction such as bubble detachment diameter, bubble detachment frequency and nucleation site density in a subcooled flow boiling. In this work, a transparent heated surface was used to obtain the data from back side of heated surface to avoid overlapping bubbles by using high speed video camera. It enabled to observe bubble nucleation process and active nucleation sites. The experiment was performed in a vertical rectangular channel at atmospheric pressure and the water was used as test fluid. In generally, the computational analysis for a subcooled flow boing deal with mean bubble size as the size of bubbles produced on heated surface. Although, it was found that mean bubble size can represent bubbles produced at same site because they are almost uniform size. Even though the size of bubbles at same site are almost uniform, the difference of the size of bubbles between other sites are considerable value. Therefore, mean bubble size on the surface should not represent bubbles for all site otherwise the serious error may be caused. It seems that bubble detachment diameter should not be given by correlations of mean bubble detachment diameter for accurate prediction of vaporization rate. Some researchers proposed that bubble size distribution should be considered by Gaussian distribution [1–3]. However, it found that bubble size distribution data accumulated in this work cannot be fitted by Gaussian distribution and there are probability that larger bubbles are neglected due to the configuration features of Gaussian distribution. So, Gamma distribution was used to predict the bubble size distribution and it was evaluated in terms of heat flux, wall superheat, mass flux and liquid subcooling. And then, by the experimental approaches, the important dimensionless parameters are identified such as Nusselt number, Jakob number, Reynolds number and dimensionless subcooling. Furthermore, vaporization rate was calculated by correlations of bubble detachment diameter, bubble detachment diameter and nucleation site density and compared with the data. Finally, the effect of using mean bubble size or bubble size distribution on vaporization rate was investigated.