Superconducting thin films of Bi-Sr-Ca-Cu-O system have been prerared on MgO substrates by RF-magnetron sputtering using Bi₁Sr₁Ca₁Cu₂O_x powder target. The as-sputtered films deposited at room temperature were amorphous insulators. After annealing, the films exhibited a superconducting transition at about 80K and characteristics of the films changed remarkably. The effect of the annealing process on superconductivity, surface condition, composition and orientation of the films were investigated.