The humidity sensitive properties of multiwalled carbon nanotubes(MWNTs) films h ave been studied.Multiwalled carbon nanotubes were synthesized by hot filament chemical vapor deposition.Carboxyl modified and non_modified MWNTs film's tempera ture effect and humidity sensitive behavior were investigated separately.The res ults showed that carboxyl modified MWNTs films were very sensitive to temperatur e and humidity,response and restored time were much faster than that of non_modi fied MWNTs films.The origin of humidity sensitivity of MWNTs films were also discussed,and the theoretical formula was given.