The present invention provides a miniature gas sensor, which comprises a gas sensor chip. The gas sensor chip includes a hollow structure on the back. An insulating layer is disposed below the sensing material. A miniature heating device is disposed surrounding the sensing material. The sensing material is adhered to the sensing electrodes. The sensing material includes two metal oxide semiconductors or a compound structure of the sensing layer having a metal oxide semiconductor and a reaction layer with a rough surface. An interface layer is sandwiched between the two metal oxide layers for increasing the efficiency in sensing gas. The gas sensor according to the present invention can be implemented on silicon substrate with hollow structures. In addition, the size of the chip can be miniaturized.