La-doped n-type ZnO and SnO2 thin films are prepared by method of vacuum evaporation on glass substrates.The effects of doping content and heat-oxidation on the structure

surface topography and sensitive characteristics are investigated.The results show that surface is refined after La-doping.With the increasing doping amount

the average grain size of ZnO and SnO2 thin films increased as well.In addition

the operating temperature for gas sensing is reduced significantly by the way of La-doping.In contrast with SnO2 doping

the La-doped ZnO films show better gas sensing property.When environed in reducing gas environment（acetone

ethanol）with volume fraction of 6×10-3

the sensitivities to acetone and ethanol of 3% La-doped ZnO thin films are 36 and 23;the sensitivity to acetone of 3% La-doped SnO2 thin films is 15.The 5% La-doped SnO2 films show the improvement of the sensing property to ethanol.