The two-phase system approach is a method to determine the degree of crystallinity without a need for a stander. The method applies the crystallite theory of amorphous models and uses the great potentiality of Rietveld analysis of multiphase systems considering the crystalline and amorphous counterpart as two different phases. It was verified, first, by artificial mixtures using simulated X-ray diffraction patterns and then tested by experimental diffraction patterns obtained from synthetic mixtures of crystalline and amorphous SiO2 with different ratios. The models for the crystalline and crystalline counterpart of the amorphous phases were, respectively, considered to have the hexagonal space group (P312) and the cubic space group (P213). The proposed approach was used for the determination of the degree of crystallinity and can be considered as standarless method free from the effect of the matrix absorption and the improbable background estimation. Also, higher accuracy was achieved comparing with those calculated by using crystalline index method. In addition to crystallinity determination, the method gives a useful structural picture of crystalline and amorphous counterparts at the same time.