We have studied the polytypism induced by the chemical potential through an ab-initio-thermodynamiccombined approach in Au-catalyzed GaAs nanowire growth. The nucleation in nanowire growth is numerically discussed by classical nucleation theory parameterized by ab initio calculations on the formation energies of the GaAs surfaces and steps. By using the approach we found that the polytypism induced by chemical potential could be explained by the energy competition of lateral walls of the nucleus very well. Furthermore, the obtained results confirmed that the driving force of wurtzite phase nucleation originates from the lower formation energy of its (10 - 10) surface. (C) 2015 Elsevier B.V. All rights reserved.