It has been a long-standing challenge to produce air-stable few-layer black phosphorus (BP) because BP degrade rapidly in ambient atmosphere. Here we demonstrate that the pristine BP with ten layers or more possesses a strong stability and can be stored in air for two weeks. The physical mechanism can be ascribed to the native phosphorus oxide formed on the surface, which act as a stable and protective capping layer and prevent the underlying layer from further oxidation. The pristine 10-layer BP FET device can maintain a high mobility value of around 220 cm2 V-1s−1 in air for 2 weeks. By contrast, the plasma-induced phosphorus oxide is not dense or robust enough to protect the underlying sample from oxidation. These results suggest that the density of the oxide layer on the surface plays a vital role in the stability of BP flakes. This work offers new perspectives for probing the stability of BP based electronic and optoelectronic devices.