Tianmu glaze wares are a very famous form of ancient Chinese porcelain. The samples were prepared with local raw materials and traditional technology. X-ray diffraction, scanning electron microscopy, optical microscopy and energy dispersive spectroscopy were used to analyse the phase composition, microstructure and chemical composition of the Tianmu glaze. The results show that the patterns were created by the process that includes phase separation, the iron-rich phase moving to the surface, the growth of nanosized magnetite and hematite crystals, the aggregation of a thin crystal layer, and the appearance and collapse of bubbles. Scattering, diffraction and interference of nanosized grains are the primary colour rendering mechanism.