Based on density functional first principles method, the defect properties in tetragonal BaTiO3 have been studied. The results showed that the formation energies of neutral Ti vacancy and partial Schottky defect 2V3-Ti+3V2+Oare the lowest under oxygen-rich condition; while under reducing condition oxygen vacancy becomes the primary defect. The calculated full Schottky formation energy is higher than that obtained in the cubic phase, which may be the result of the strong hybridization between the Ti-O bonds. The hybridization is also responsible for the Frenkel formation energy of Ti. The defect-interactions are important when dealing with Schottky defects.