Surface energy has an important role in determining the properties of nanoparticles. Even though, extensive research has been done on the surface energy of pure nanoparticles, the surface energy of alloy nanoparticles has not been considered enough. In this work, based on the liquid drop model for surface energy of pure nanoparticles and Butler's equation, a model for size dependent surface energy of alloy nanoparticles has been developed. In addition to size and concentration, the model can describe the effects of shape and temperature on surface energy of alloy nanoparticles. Cu - Ag and Cu - Au systems have been studied as two examples and the results have been compared with other theoretical models and available simulated data. Reasonable agreements between the results were observed. The results show that the decreasing particle size decreases surface energy of alloy nanoparticles but decreasing temperature and shape factor increases the value of surface energy. (C) 2017 Elsevier B.V. All rights reserved.