We measured the Raman spectra of CO2 fluid at 30 MPa and room temperature (19.1-21.3°C). The spectra showed peaks attributed to the CO2 combining various isotopes such as 12 C 16 O2, 13 C 16 O2, and 12 C 16 O 18 O. The relative ratio of 12 C 16 O2 to 12 C 16 O 18 O peaks represents the oxygen isotope ratio of CO2. To evaluate the precision of the peak ratios, we measured the CO2 Raman spectra at different exposure times. We examined the standard deviation (1 ) of the peak intensity ratios and area ones for 20 measurements at each exposure time. The standard deviations of the intensity ratios and area ones were 2.7 and 3.1%, respectively, at a maximum exposure time of 494 s, conversion to peak intensity of 12 C 16 O2 yields about 2,000,000 counts. The uncertainties are 2.5-3 times greater than expected from the noise of a CCD camera and photon statistics. The oxygen isotope ratios ( 18 O) of natural samples have a range of variation of about 16.6%.Compared to that value, the precision we obtained from this study is very small. Raman spectroscopy can be combined with microscopy to analyze areas as small as approx. 1 m in diameter. Therefore, oxygen isotope measurement using Raman spectroscopy has potential for application to natural samples as a new method for small CO2 fluids such as fluid inclusions.