This paper reports on the development of a machining device which is capable of carrying out precision machining experiments under external hydrostatic pressure. Machining trials were conducted on hard-brittle materials such as soda glass, quartz glass, silicon and quartz wafers using the newly developed machining device under the externally applied hydrostatic pressures of zero and 400 MPa. The machined traces were analyzed by laser microscope. From the trace profiles, crack ratio and area of cross section of the trace were estimated. The applied hydrostatic pressure enhanced the critical cross sectional area and reduced the cracks and chippings of all the tested materials. Effects of hydrostatic pressure on the machining characteristics of the crystalline and glassy materials are discussed in detail. The mechanism behind the enhancement of ductile-brittle transition by the externally applied hydrostatic pressure is also elucidated by a theoretical model.