The nucleation mechanism of zeolite A is investigated by means of Density Functional Theory (DFT) calculations. We calculated the Gibbs free energy change for the polymerization and cyclization reactions involved in the nucleation of zeolite A in the gas phase and solution between 298 and 450 K. Our analysis reveals that the four-ring species formed could be the most likely to participate in the nucleation of zeolite A, and its nucleation mechanism could proceed by a reaction route which involves the formation of the double-four-ring.