In this study, a series of ZnO thin films were grown successively by hydrothermal method, and their structural, morphological and CO2 gas sensing properties were investigated depending on the zinc concentration in the range between 0.01 and 0.08 M. Average crystallite size of the samples was between 65 and 60 nm, which were estimated by Debye-Scherrer formulation, these values decreased by the increasing zinc concentration. The resistance of the films decreased from 2.3 x 10(10) omega to 1.1 x 10(9) omega by increasing zinc concentration. The maximum CO2 gas response increased from 87 to 234%, when concentration changing from 0.01 to 0.06 M, then the response began to decrease for 0.07 M. The maximum response was calculated as 234% for 50 ppm CO2 gas at 190 degrees C temperature. Gas sensing results of 0.06 M towards 100 ppb CO2 gas were indicated that good stability, reducibility and repeatability were determined. When the CO2 sensing mechanism was investigated in detail, suggesting that increasing zinc concentration had an effect on the particle size, surface images and thereby improved CO2 gas sensing mechanism.