A thermodynamic model based on Gibbs free energy and availability is developed for onset of heterogeneous nucleation on heated surfaces with different wettabilities in pool boiling. Different from classical nucleation theory, this model takes into consideration the temperature gradient in the superheated liquid layer adjacent to the wall as well as the contact angle between the liquid and the wall. Using Gibbs free energy equilibrium condition, a closed form solution is obtained on the critical radius for onset of heterogeneous boiling nucleation on walls with different wettabilities. Effects of contact angles and wall temperatures on the critical radius, the wall temperature gradient of the superheated liquid layer and the heat flux at onset of heterogeneous nucleate boiling are illustrated. These effects on the change of availability during the heterogeneous nucleation process, representing the energy barrier for the occurrence of the first-order phase transition, are also discussed. (C) 2011 Elsevier Ltd. All rights reserved.