The main color in the glaze of most Jun ware is a large range of blues. In order to investigate the origin of the blue glaze color and its change in hue, seven pieces of ancient Jun ware were analyzed using scanning electron microscopy, transmission electron microscopy, Mossbauer spectroscopy and Ultraviolet-visible spectrometry. The results show that Fe²⁺ is the dominant colorant in the Jun glaze and that the higher the Fe²⁺ concentration, the higher the saturation of the blue color. Phase separation is also observed in the Jun ware glazes. The big droplets endow the glaze with a greenish hue with higher luminosity, which finally weakens the iron blue color. The blue color of Jun ware glaze is a result of both its chemical composition and the microstructure, in other words, a consequence of both the absorption and scattering of the light.