The present invention provides a gas sensor structure comprising a gas sensing chip. The back of the sensing material is a hollow structure. An insulating layer is below the sensing material. A micro heating is disposed surrounding the sensing material. The sensing material adheres to sensing electrodes. The sensing material is a complex structure including a metal oxide semiconductor and a roughened lanthanum-carbonate gas sensing layer. The thickness of the metal oxide semiconductor is between 0.2 μm and 10 μm; the thickness of the roughened lanthanum-carbonate gas sensing layer is between 0.1 μm and 4 μm; and the size of the back etching holes is smaller than 1*1 mm. By using the gas sensor structure according to the present invention, a suspended gas sensing structure can be fabricated on a silicon substrate and the chip size can be minimized.