Two-dimensional (2D) materials have aroused considerable research interest owing to their potential applications in nanoelectronics and optoelectronics. Thermodynamic stability of 2D structures inevitably affects the performance and power consumption of the fabricated nanodevices. Black arsenic (b-As), as a cousin of black phosphorus, has presented extremely high anisotropy in physical properties. However, systematic research on structural stability of b-As is still lacking. Herein, we demonstrated the detailed analysis on structural metastability of the natural b-As, and determined its existence conditions in terms of two essential thermodynamic variables, hydrostatic pressure and temperature. Our results confirmed that b-As can survive only below 0.7 GPa, and then irreversibly transforms to gray arsenic, consistent with our theoretical calculations. Furthermore, a thermal annealing strategy was developed to precisely control the thickness of the b-As flake, and it sublimates at 300 °C. These results could pave the way for 2D b-As in many promising applications.