Montmorillonite is a widely distributed clay mineral in the oil and gas basin. When it is encountered with water, its volume can expand by several or tens of times, which can be ascribed to the strong hydrophilic character. Consequently, oil and gas reservoirs might be damaged by its water-sensitive property. In this study, the phase transformation and hydrophilic behaviors of natural Na-smectite were investigated in alkaline solution. The natural smectite was put in solution of 0.1 M KOH, 0.5 M KOH and 1 M KOH at 80℃ for 30 days. The pH values of KOH solution after reaction were measured, and solid products were analyzed by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), and electron dispersive analysis of X-rays (EDAX). The XRD results indicated that part of montmorillonite was transformed into chabazite and merlinoite after 30 days. The results showed that phase change of montmorillonite was related to the concentration of KOH and reaction temperature. Concomitantly, new crystals in plate-like, fibrous and granular forms were observed by SEM. Accom-panied by the phase transformation, the hydrophilicity and the free swelling volume of minerals were reduced.