We have developed a microsensor using a suspended microbridge and a carbon nanotube networks (CNN) that is in-situ synthesized on the microbridge for detection of trace energetic substances. The energetic substance vapors are adsorbed onto the CNN surfaces and are heated to micro deflagration using an integrated heater on the microbridge. The involved temperature changes are measured with an integrated thermistor to obtain the thermal spectra for vapor discrimination. The huge surface areas of the CNNs significantly improve the adsorption efficiency and the limit of detection (LOD), and the tiny thermal mass allows distinct temperature changes to be generated. A LOD of 13 pg has been achieved for 2,4,6-trinitrotoluene (TNT) detection.