Vapor bubbles attached to the heated surface in a subcooled boiling flow usually reach their maximum size during the latter phase of the bubble growth period when the liquid microlayer trapped under them is almost depleted. The heat transfer at the bubble during this phase involves only the transient heat conduction through a so-called relaxation microlayer surrounding the lower bubble surface and the condensation at the bubble dome. On this physical base, a new mechanistic model for predicting the maximum diameter of attached vapor bubbles in a subcooled boiling flow is proposed in this study. The new model is derived from the lumped energy balance for the bubbles. It is then validated using published experimental databases on the maximum bubble diameter measured for subcooled boiling flows of water under a wide range of flow conditions. A good agreement between the predicted maximum bubble diameter and the experimental one is obtained. The average relative error is less than about 35.5%. This model is expectedly worthy of being used in the analysis of subcooled boiling flows. (C) 2015 Elsevier Ltd. All rights reserved.