The present invention relates to a process for vapor depositing a low dielectric insulating film, a thin film transistor using the same, and a preparation method thereof, and more particularly to a process for vapor deposition of lowdielectric insulating film that can significantly improve a vapor deposition speed while maintaining properties of the low dielectric insulating film, thereby solving parasitic capacitance problems to realize a high aperture ratio structure, and can reduce a process time by using silane gas when vapor depositing an insulating film by a CVD or PECVD method to form a protection film for a semiconductor device. The present invention also relates to a thin film transistor using the process and preparation method thereof.