Stable carbon isotope composition (δ¹³C value) was one of the effective methods for measuring the plant water use efficiency (WUE),and was already applied in many plants. At present, the study on estimation of the WUE of Cassia obtusifolia L.. by δ¹³C was a few. In order to prove the reliability of δ¹³C on measuring WUE for Cassia obtusifolia L., δ¹³C、stable carbon isotope discrimination (Δ¹³C)、WUE and the relationship among them are measured in different organs (root, steam, leaf) and different growth stages by pot experiment and δ¹³C. The results showed that, the δ¹³C value and Δ¹³C of roots were highest, the next were stems, leaves were minimum, there were significant level in the δ¹³C value and Δ¹³C in roots in different growth stages, positive relationship were found between the δ¹³C value and WUE in stems and leaves, negative relationship were found between the Δ¹³C and WUE in stems and leaves. It showed significant positive correlation level among WUE (Potted method) and WUE_L、WUE_S、WUE_R (Carbon isotope method), related coefficient were 0.86、0.82、0.80 respectively, those showed carbon isotope technology was reliable to determine the WUE of Cassia obtusifolia L..