Highly transparent conducting indium tin oxide (ITO) films have been prepared on polyester thin film substrate by reactively evaporating metal Ln-Sn alloy in a system with an oxygen partial pressure of (3-30) X 10(-5) Torr and substrate temperatures between 80 and 240 degrees C. The structure and opto-electrical properties of the films depending on the deposition conditions have been investigated. High quality films with a average transparency of 83% and the resistivity of 7 X 10(-5) Ohm cm have been obtained by controlling the deposition parameters.