Abstract: The SnO_2 ultralfine particle films(UPF)were prepared by a method called D. C.gas discharge activating reacting deposition. The complex impedance spectroscopy of the films were measured.It is proved that the proposed equivalent circuit of the UPF was feasible,The effects of temperature,reducing gas,water vapour and heat treatment on impedance spectroscopy have been discussed.These phenomena have been explained using the model of oxygen's adsorption and theories of carriers to cross intergrain potential barrier. All of these are helpful to the farther research of gas sensing mechanisms and conduction mechanisms of the UPF.