The leakage current and relative permittivity of (Ba0.5Sr0.5)TiO3 (BST) thin films prepared by pulsed-laser deposition (PLD) were investigated. It was found that the leakage current for positive bias voltage was higher than that for negative bias voltage, which was attributed to the lattice mismatch between the bottom Pt electrode and the BST thin film. A time-dependent breakdown process under positive voltage was observed, which was interpreted as the increase of the internal electric field in the film near the bottom electrode. However, the internal electric field decreased and could eventually be recovered by applying negative bias voltage.