Ba 0.8Sr 0.2TiO 3 thin films were deposited on platinized Si substrates by pulse laser deposition (PLD) under different O 2 residual pressure, and the effect of prolonged thermal treatment on the films was investigated. It was found that both surface roughness and oxygen vacancy affect the conductance of the capacitor. The prolonged thermal treatment does not change surface roughness of 10-Pa deposited film too much, leading to a decrease of leakage current by eliminating oxygen vacancy, whereas it strongly increases surface roughness of 20-Pa deposited film, resulting in a slight rise of leakage current in contrast. Best leakage current properties were achieved in annealed 10-Pa deposited film with forward leakage current of 2.42×10 −8 A/cm 2 and reverse leakage current of 2.51×10 −9 A/cm 2 under 1.5 V bias.