As a promising anode material alternative to the carbons, Li4Ti5O12 has drawn wide attention for its overwhelming advantages in meeting requirements of lithium ion batteries with high power density, long cycling life and safety for electric vehicles and electricity storage stations. Its stability issue during storage is interesting and of large importance. In this work, the stability of commercial spinel Li4Ti5O12 material in air has been characterized by a series of physical and electrochemical techniques. First-principles calculations indicate that Li4Ti5O12 is lithium-truncated at the surface. This explains why it tends to absorb H2O and CO2 to form Li2CO3 but the formation of Li2CO3 does not significantly impact its electrochemical performances.