We fabricated transparent composite films which have good thermal stability, low CTE (coefficient of thermal expansion) and high mechanical properties for flexible OLED substrate. Glass fiber reinforced composite films were made by UV light induced chemical crosslinking of composite materials consisted of glass fiber and UV curable resin in film state. It was noted that the pattern of the reinforce glass fiber had important effect on the optical and mechanical properties of the resulting composite film. The performance of the composite films in the ITO thin film deposition process and subsequent OLED device fabrication was better than those of the known polymer films such as PET, PC, and PES.