In order to study quartz-coesite transition under the conditions of differential stress, experiments of quartzite deformation were carried out using a triaxial testing system with a Griggs type solid medium pressure vessel. Analyses on the plastically-deformed samples under optical microscope and Raman spectra show that fine-grained coe-site was present in the region of samples adjacent to the pistons at temperatures of 950-1000℃, confining pressure of 1.3 GPa, differential stress of 1.5-1.67 GPa, and total strain of 75%-81%. It is evident that the transition pressure of quartz-coesite at differential stress and intensely-strained conditions is far lower than the pressure for coesite stability at isostatic pressure. In other words, the stress condition of coesite occurrence is not unique. The decrease in confining pressure for quartz-coesite transition under differential stress conditions is controlled by a combined effect of the maximum principal stress that provides a high stress environment, and diff In order to study quartz-coesite transition under the conditions of differential stress, experiments of quartzite deformation were carried out using a triaxial testing system with a Griggs type solid medium pressure vessel. Analyzes on the plastically-deformed samples under optical microscope and Raman spectra show that fine-grained coe-site was present in the region of samples adjacent to the pistons at temperatures of 950-1000 ° C, confining pressure of 1.3 GPa, differential stress of 1.5-1.67 GPa, and total strain of 75% -81%. It is evident that the transition pressure of quartz-coesite at differential stress and intensely-strained conditions is far lower than the pressure for coesite stability at isostatic pressure. In other words, the stress condition of coesite occurrence is not unique. The decrease in confining pressure for quartz-coesite transition under differential stress conditions is controlled by a combined effect of the maximum principal stress that provides a high stress e nvironment, and diff