In order to establish a reliable remaining life assessment method for high temperature components in thermal power plants, it is necessary to understand the internal damage mechanism in heat resistant alloys and establish the damage prediction method. In the case of commercial heat resistant alloy used for thermal plants, nucleation, growth and coalescence of creep voids are dominant damage mechanism. So far, the void growth mechanism is well investigated and various growth models are suggested. However, rational modeling of void nucleation was remained to be difficult subject because basic mechanism has been unclear. Therefore, in this study, void nucleation behavior of a turbine rotor material under creep condition was investigated by a scanning electron microscope and development of void nucleation models was examined. Void nucleation model was developed based on the idea that creep voids nucleate by vacancy condensation at the interface between precipitated carbides and matrix on the grain boundary. In addition, consideration of creep property was included in the model via a relationship between strain and vacancy density obtained from the molecular mechanics simulation. In addition, model was expanded to predict the void number density, by a statistical treatment of the different physical property related to void nucleation in the every grain boundary. It was confirmed that the present void nucleation model can reproduce evolution of void number density.