Formaldehyde gas sensors with a structure of ceramic tube were fabricated by using La0.8Sr0.2FeO3-doped nanometer tin oxide. The highest response to formaldehyde appeared when the heating temperature of the La0.8Sr0.2FeO3-doped SnO2 sensor was about 370 °C. The response of the sensors to formaldehyde was measured in a gas concentration in the range of 0 - 5 ppm. The lowest formaldehyde concentration detected by 2 wt % (weight ratio) of La0.8Sr0.2FeO3-doped SnO2 gas sensor was 0.05 ppm. The response and recover times of the sensor to 0.5 ppm formaldehyde were about 120 s and 100 s, respectively. The response of the sensor to some interferent gases was tested.