BST thin films have been fabricated by RF magnetron sputtering onto Ir layer as a bottom electrode. When the substrate temperature was maintained at 600 °C during deposition, BST films deposited at that temperature showed very small oxide equivalent thickness of 0.36nm as well as very low leakage current density of about 10−8A/cm2 at 1.5V. But as substrate temperature was increased to 700 °C in order to obtain high dielectric constant, oxide equivalent thickness exhibited very low value of about 0.3nm, however, leakage current density drastically increased to 10−4/cm2. BST thin films were fabricated by two step process, which consists of bottom layer deposited at high substrate temperature of 700 °C and top layer deposited at low substrate temperature. In the case of BST thin films which are composed of 20nm thick bottom layer deposited at 700 °C and 30nm thick top layer deposited at 500 °C, we obtained very small oxide equivalent thickness of 0.31nm and low leakage current density of 4 × 10−8 A/cm2. at 1.5V.