High-quality ITO films on flexible PET substrate were prepared by RF magnetron sputtering at low deposition temperature with different Ar gas sputtering pressure. Adhesion and electro-optical properties of ITO films were investigated as a function of Ar partial pressure. The sputtering conditions provide very uniform ITO films with high transparency (> 85% in 400–760 nm spectra) and low electrical resistivity (1. 408 × 10 −3 −1.956 × 10 −3 Ω · cm). Scratch test experiments indicate that there is a good adhesion property between ITO films and PET substrate, the critical characteristic load increases from 16.5 to 23.2 N with increasing Ar sputtering pressure from 0.2 to 1.4 Pa.