Carbon and oxygen isotope ratios ( 13 C and 18 O) of calcium carbonate samples are ordinarily translated their raw measurement results into PDB scale by using a working standard, which isotope ratio is calibrated by an international carbonate standard. This study reports the isotope values of carbonate standard materials of JCp-1 and JCt-1 and evaluated them as the working standards of the isotope measurements. The isotope ratios of repeated measurement showed uniform values, 13 C PDB =

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... 03‰, 18 O PDB =-4.71±0.03‰ for JCp-1, and 13 C PDB =+2.79±0.02‰, 18 O PDB =-0.496±0.03‰ for JCt-1, and they are appropriate for working standards. We can determine isotope value of sample, which isotope range is -5.01‰< 13 C PDB <1.95‰ and -22.97‰< 18 O PDB < -2.20‰, by using a singlepoint anchoring of JCp-1 or JCt-1.