The Cretaceous rocks which contain glauconite at different stages of glauconitization lie under the glauconitic sandstone beds in the Xiala section in southwestern Tibet. To better understand the glauconitization process as well as the mechanism and factors that controlled the formation of glauconite, the authors studied the glauconite at different stages of glauconitization in the Cretaceous rocks of the Xiala section by means of petrographical microscope, Scanning Electron Microscope (SEM), and the Electron Probe Micro-Analyzer (EPMA). The results show that the glauconite generally occurs as granular aggregates composed of lamellae on the micron scale and has evident characteristics of replacement. The replacement of K in alkali feldspar dominated the interface migration reaction, which was controlled by a coupled dissolution-precipitation process, and a later stage of recrystallization. A long-term low-energy environment was necessary for the newly formed glauconite to retain the shapes of original mineral grains. Three factors were crucial at the initial stages of glauconitization: supply rates of sediment, dissolution rates of the substrate, and the redox condition near the reaction surface. The widespread occurrence of berthierine indicates that reducing condition was prevailed, and that glauconite only developed under local suboxic condition. The relatively high K₂O content and low TFeO content of glauconite at different stages of glauconitization indicate that the process of glauconitization in the Xiala section could not be explained by the neoformation theory or the two-stage model postulated by previous researchers, because unlike the glauconitization of K-deficient minerals, the process in the Xiala section did not require a second stage of K enrichment.