In this study, we proposed a new carbon dioxide (CO2) gas sensor based on an ionic liquid filled photonic crystal fiber (PCF). 1-ethyl-3-methylimidazolium tetra fluoroborate (EMIMBF4) was selected to fill the PCF as the ionic liquid because carbon dioxide can easily dissolve in it. Optical transmission differences, depending on the carbon dioxide in the medium, was investigated. The optical transmision spectrum was measured between wavelengths of 500 to 850 nm. It was observed that the light transmission decreased under the influence of CO2; however the decrease was considerably less between wavelengths of 650 to 850 nm. The transmission changes observed under the influence of CO2 at 550 and 850 nm were %60 and %28 respectively.