The controlled fabrication of metal oxide nanocrystals with exposed high-index facets has drawn enormous research attention. In this article, we have synthesized single-crystalline a-Fe2O3 crystals bound by six {104} high-index facets using a simple hydrothermal method. The formation mechanism of the unique crystal structure has been systematically studied, revealing that the a-Fe2O3 crystals are formed through an aggregation-phase transformation-recrystallization mechanism. More importantly, we have demonstrated that the unique a-Fe2O3 crystals with {104} high-index facets are highly advantageous for high performance gas sensors. This work sheds some new light on using high-index faceted nanocrystals for high performance functional device applications.