ZnO micro/nanocrystals with small size and more exposed high-surface-energy facets usually exhibit better photocatalytic properties. However, the stability of ZnO micro/nanocrystals used as a photocatalyst is always ignored. In this work, ZnO micro/nanocrystals with different percentages of exposed (0 0 0 1) facets were employed as a photocatalyst in the degradation of methylene blue (MB) and their stability dependent on the exposed facets and grain size was discussed in detail. The higher percentages of exposed (0 0 0 1) facets can be obtained at a larger volume ratio of water/methanol (H2O/CH3OH) in the chemical bath method. It was found that ZnO micro/nanocrystals with a higher percentage of exposed (0 0 0 1) facets showed larger changes in the exposed (0 0 0 1) and (0 0 0-1) facets after their photodegradation of MB in aqueous solution, indicating a poor stability, although they displayed a better photocatalytic performance. Interestingly, ZnO nanocrystals with small size exhibited no obvious changes in their shape after their application as a photocatalyst. In this sense, exposed facets played more important roles in the stability of ZnO micro/nanocrystals than crystal size. This may be because that the Zn and O atoms in the exposed (0 0 0 1) and (0 0 0-1) facets preferred to be participated in the photocatalytic reaction and then formed Zn2+ ions and oxygen species in the solution after the reaction. Therefore, this work can provide a new insight into the stability of metal oxide micro/nanocrystals as photocatalysts.