Radish striae constitute an important symbol for identification of Tianhuang. By means of rock slice， XRD test and electron microprobe analysis, the author found that the mineral constituent of the radish striae is illite. The formation of radish striae lies in the mechanism that microcracks were formed in the primary rock of Tianhuang under tectonic stress and later filled with heterogeneous illite. The illite in the microcracks looks like fibre （radish striae） because of the obvious optical difference between illite in the microcracks and dickite as the main mineral constituent in the rock.