There are provided a micro gas sensor and a method for fabricating the same that comprises a micro heater formed inside a polysilicon membrane by doping impurities into a specific region of the polysilicon membrane positioned under a gas sensing substance, thereby improving thermal structural stability and making it easy to form the gas sensing substance. The micro gas sensor comprises: a micro heater formed by doping impurities into polysilicon vapor-deposited on a substrate on which a first insulating layer is formed; a polysilicon membrane for decreasing a heat loss of the micro heater; a power electrode for supplying power and a temperature measurement electrode for measuring a temperature, positioned at both ends of the micro heater; a second insulating layer formed on the micro heater; a sensing substance formed on the second insulating layer, for sensing a gas; and a sensing electrode for measuring a change in properties of the sensing substance. The method for fabricating a micro gas sensor comprises steps of: forming polysilicon on a substrate on which a first insulating layer is formed; forming a micro heater by doping impurities into the polysilicon; forming electrodes at both ends of the micro heater; forming a second insulating layer on the micro heater; forming a sensing substance on the second insulating layer; and forming a sensing electrode on the sensing substance.