We determined the electron density trapped at the Pt/BST interface in Pt/(Ba,Sr)TiO3/Pt MIM capacitor. The electron densities trapped at the top-Pt/BST interface and the bottom-Pt/PST interface were calculated to be constant (2 similar to 3 x 10(12) cm(-2)) for all the as-deposited films of varying thicknesses. Rapid thermal annealing (RTA) was attempted to reduce the interface trapped electron density of MIM capacitor. The MIM capacitors were post-annealed at 650 degrees C for 30 seconds in oxygen and nitrogen atmosphere. As a result, the trapped electron densities were reduced to one third compared with those of the as-deposited samples. And leakage current density of the annealed samples also decreased drastically. So the rapid thermal annealing process is thought to be very effective in reducing leakage current densities of BST MIM capacitors.