The effects of microbubble injection on natural convection heat transfer from a vertical heated plate in water are experimentally investigated. Thermocouples are used to measure the temperature, and an image processing technique is used to obtain the bubble diameter, bubble velocity, and bubble layer thickness. Tap water is used as the working liquid, and hydrogen bubbles generated by water electrolysis are used as the microbubbles. At a constant wall heat flux, microbubble injection significantly increases the heat transfer coefficient in both the laminar and transition regions. The ratio of the heat transfer coefficient with bubble injection to that without injection is 1.6-2.0 in the laminar region and 1.5-2.0 in the transition region. The enhancement of heat transfer in the laminar region results from both the forced convection and mixing effects. In contrast, the enhancement of heat transfer in the transition region arises mainly because microbubble injection accelerates the transition to turbulence. (C) 2013 Elsevier Ltd. All rights reserved.