This paper introduces a new damage free machining method for hard-brittle materials; machining under high hydrostatic pressure. A new machining equipment, in which scratching test can be performed under high hydrostatic pressure of 400MPa, was developed. Silicon wafer and silicate glass were applied to this equipment to evaluate effects of hydrostatic pressure on their ductile-brittle transition. Results show drastic decrease of machining defect and increase of critical groove depth by hydrostatic pressure. This paper also discusses origins of machining defect, and indicates that defects are attributed not on microcracks on the surface of workpieces but those in workpieces. It also points out a problem of the initial crack model of machining defect generation.