The retention properties of SrBi2Ta2O9 (SBT) thin films are to be studied. Within the first second, the polarization decay increases with increasing of the write/read voltage, and tends to steady value. This could be ascribed to the depolarization fields, which increases with increasing retained polarization. However, the polarization loss is found to be different with various write/read voltages over a range of 1-30 000 s. The effect of ultra-violet irradiation on SET retention is also to be investigated. Experiments indicate that there was weak pinning of domain walls existing in SET.