X-ray diffraction experiments were carried out with protoenstatite, chemical composition Mg2Si2O6, in order to clarify the conditions under which protoenstatite can be retained at room temperature. Our results show that grain size, cooling rate, and shear stress during sample preparation clearly affect the transition from protoenstatite to clinoenstatite. Smaller protoenstatite grains were more likely to be retained, and the relationship between the retained volume ratio of the protoenstatite and grain size was statistically consistent with martensitic nucleation. The most protoenstatite was retained in the experiment using a cooling rate of 3 °C/min; the retained volume ratio decreased in experiments with both faster and slower cooling rates. The martensitic transformation of protoenstatite to clinoenstatite is promoted by shear stress caused by a fast cooling rate. Shear stress caused by grinding and polishing also promotes the transformation, but ion milling, used to prepare samples for transmission electron microscope observation, leaves the protoenstatite unchanged. Therefore, samples including protoenstatite should be prepared without producing shear stress so that the protoenstatite can be observed.