The quantification of amorphous phase can not be obtained by Rietveld. As the nonlinear mapping relationshipcan be carry out by BP neural network, the content and the background of the amorphous phase have corresponding relation, and their relationship is determined. The data of the background and the diffraction peak area was calculated with BP neural network to acquire the content of the amorphous phase. The calculated result of BP neural networks is better than that of polynomial and sigmoidal function. At the same time, the applicability of BP neural network calculation is also given.