The electrical instability of single-walled carbon nanotube (SWCNT) network-based thin-film transistors is investigated in atmospheric air and under vacuum. Atomic force microscopy images show that the nanotube bundles form X-type and Y-type nodes in the SWCNT-networkfilm. The Raman spectrum reveals that the structural defects in the SWCNTs are negligible. The fabricated SWCNT-network TFTs operate in a p-channel accumulation mode both in air and under vacuum. In contrast, TFTs exposed to atmospheric air environment exhibit lower drain currents and larger hysteresis compared with the vacuum environment case. An analysis of the time-dependent characteristic degradation of the SWCNT-network TFTs also demonstrates that the initial decay of the drain current in atmospheric air environment is more acute than that under vacuum. These results can be explained in terms of the hole-trapping behavior of the water molecules near the nanotubes or at the SWCNT/dielectric interface as well as the compensation effect of the electrons donated by water molecules with free holes in the SWCNT-networkfilm.