Carbonate δ13C data reflecting the primary marine isotope compositions, are crucial for the success of C-isotope chemo. Based on many case studies, two important steps are proposed to evaluate credible isotope data in the study of C-isotope chemo stratigraphy. The first step is to carefully select samples for isotope analysis. The second step is to evaluate the validity of δ13C data using elemental geochemical data. Commonly used methods include δ13C and δ18O crossplot, correlation with global secular variation, comparison with C-isotope profiles from other coeval sections,correlatio between δ13C and Mn/Sr, and correlation between δ13C and Fe/Sr.