Stable n-type thin film transistors (TFTs) are fabricated with Cs encapsulated single-walled carbon nanotubes (Cs@SWNTs). The transport property of SWNTs-TFTs clearly changes from p- to n-type characteristic after the Cs plasma irradiation. Based on the systematic investigations, it is revealed that there is an optimum ion energy for the effective Cs encapsulation, which is around 50 eV. Furthermore, it is also found that the n-type feature is stable even in water and high temperature (≤ 400 °C) conditions. This very stable n-type TFTs is important for the practical application of SWNTs-based thin film electronics.