First-principles density functional theory plus U study on the formation enthalpy of BiFeO3 and the intrinsic vacancies was performed. The formation enthalpy of BiFeO3 from oxides is only -0.2 eV, indicating that BiFeO3 could easily decompose into Bi2O3 and Fe2O3 under thermal or electrical stresses. It is found that the vacancy induced local distortions have insignificant effect on the ferroelectric property, thanks to the high stability of the ferroelectric configuration in BiFeO3. Moreover, Bi and Fe vacancies have comparable formation energies, and become dominant in the oxygen rich conditions, leading to p-type conductivity.