Abstract: Coesite was produced at certain differential stress conditions below its stability field under static pressure in laboratary．which shows that differential stress has obvious effect on the quartz-coesite trailsition pressure．According to the forming conditions of coesite。there are two different physical mechanisms for shifting quartz-coesite phase transition boundary to decreased forming pressure at differential stress conditions：the phase transition in semibrittle deformation region due to local shear deformation induced mechanical instability，and the phase transition in plastic flow region due to presence of high dislocation density within highly deformed quartz induced strain instability.