Controlled pyrolysis of polymer ceramic precursors provides a new way of obtaining silicon nitride ceramics with high creep resistance. In this study, crack-free bulk amorphous Si–N–C materials were produced by warm-pressing followed by pyrolysis or alternatively by prepyrolysis and binding followed by pyrolysis. Amorphous compacts were then heat-treated at different temperatures to promote crystallization. High-resolution electron microscopy revealed that, at about 1650 °C, silicon nitride/silicon carbide nanocomposites with a high degree crystallinity can be achieved with grain sizes of about 30 nm. Aside from heterogeneous crystallization, which is closely related to gaseous phase reactions that happen along outer or inner surfaces, homogenous crystallization is responsible for crystallization of the bulk material. Although a certain amount of amorphous Si–N–C usually remains in intergranular regions, clean boundaries free of amorphous interlayers can be observed between grains when they come into contact.