Indirect-heat formaldehyde gas sensors were made of undoped nano-SnO_2,and doped Pd,Sb,Ti,Zr,Cu,Ag and Mn into SnO_2 respectively for improvement the sensitivity.The sen- sitivities of 2% Pd and 2% Zr-doped SnO_2 sensors are much higher than that of undoped sensor. The sensitivities of undoped,2% Pd and 2% Zr-doped SnO_2 sensors on 5×10~(-5) formaldehyde are 1.33,2.38 and 2.08 respectively.The selectivity of the sensors to ethanol can not be proved by doping.The mechanism of doped-SnO_2 sensor was analyzed....