This paper presents results of the experimental study of the subcooled boiling flow on the surface of the vertical cylindrical steel heater. The initial stages of formation of the fully developed nucleate boiling were investigated over the unsteady stepwise heat flux range from 0.56 to 1.76 MW/m(2). The field temperature of heated liquid layers at the moment of the bubble departure was described by numerical modeling under nonstationary conditions. A high-speed digital video camera was used to capture the bubble dynamics. The effects of heat release conditions and inlet water temperature on the bubble maximum diameter and nucleation frequency were studied. We found that the increased heat flux results in reduction of the maximum diameter at non-stationary heat release. Calculations on the basis of prediction models showed that the non-stationary nucleate boiling data cannot be generalized using the models developed by Prodanovic et al. (2002) and Song (2016) with the coefficients obtained for the stationary boiling. However, the heat balance approach to calculation of the bubble maximum diameter proved to be promising, and the correlation between the bubble diameter and the overheated layer thickness was verified. (C) 2018 Elsevier Ltd. All rights reserved.