Amorphous Si-B-C-N polymer-derived ceramics (PDCs) with 8.3 at.% of boron were synthesized by thermolysis of the boron-modified poly(methylvinylsilazane). The isochronal crystallization process was quantitatively studied by X-ray diffraction (XRD) measurements using variable heating rates. Crystalline structures form within the amorphous Si-B-C-N PDCs at two stages including the formation of nanocrystalline SiC (NC-SiC) at the first stage followed by the formation of nanocrystalline Si(3)N(4) (NC-Si(3)N(4)) and additional NC-SiC at the second stage. The change of the SiC crystallite size with temperature determined from the XRD analysis was used as a part of input data for the modeling. The metastable phase fraction diagrams were computed using an available model of metastable phase equilibria including amorphous and nanocrystalline phases for various modeling parameters and variable heating rates as well. The modeling performed is consistent with the experimental results to a large extent. The impact of modeling free parameters is discussed in order to explain the discrepancies observed between the experimental and computational results. The extended study of the NC-SiC formation at the first stage of crystallization justifies that this process is not purely controlled by kinetics and proves a crucial role of the metastable phase equilibrium between the amorphous Si-C-N domains and NC-SiC.