Understanding the boiling heat transfer characteristics of nanofluid suspensions containing nanoparticles is one of the challenging issues in the heat transfer research field. The addition of nanoparticles modifies both the surface characteristics and the thermo-physical properties of the suspension, including thermal conductivity, viscosity, and surface tensions. One of the critical features is the stability of nanoparticle suspension, which is strongly related to the formulation method of nanofluids. In this study, to investigate the effect of several formation methods, the pool-boiling bubble behaviors of Al2O3 nanofluid, with varying nanofluid formation methods, were systematically analyzed using an X-ray imaging system. The changes in the wettability of Al2O3 nanofluid suspensions and the variations of boiling bubble characteristics were analyzed to determine the relationship between boiling-bubble behaviors and examine the effects of heating the substrate. Before the heating substrate, the bubble generating ratio was increased by 1.4 times, when the concentration of Nanofluid 3 increased from 0.05 wt% to 1 wt% with adopting the electrostatic stabilization method and high-speed rotating procedure. On the other hand, at the end of heating process, the bubble generating ratio was decreased by 0.78 times for the same concentration change. Once the nanoparticle sedimentation occurred, the trend of wettability and bubble behaviors of nanofluids changed based on the surface modification, regardless of the nanofluid formation method. An increase in the Al2O3 nanofluid concentration results in a deterioration of the heat transfer characteristics, and the formulation method is not related to the boiling heat transfer performance when nanoparticles are deposited. (C) 2019 Elsevier Ltd. All rights reserved.