A conventional gas sensor is a metal-oxide semiconductor gas sensor, and the metal-oxide semiconductor gas sensor senses gas by a metal-oxide semiconductor powder therein, such as tin (IV) oxide powder and zinc oxide powder. In such a way, when the metal-oxide semiconductor gas sensor is at a working temperature (e.g. 200° C. to 400° C.), the metal-oxide semiconductor powder can absorb the sensed gas, resulting in a voltage change of the metal-oxide semiconductor gas sensor and a determination of the concentration of the sensed gas. However, the metal-oxide semiconductor powder occupies a large volume and has a low surface area, which leads to a low contact area for the sensed gas so that the metal-oxide semiconductor gas sensor has a low gas sensitivity. In addition, the metal-oxide semiconductor powder also has a low identification for the sensed gas and other gas, and consequently the final result by the sensor is uncertain