Objective To evaluate the clinical application of Hb-A2 detection of thalassemia including-alpha-thalassemia and beta-thalassemia. Methods There were 70 Southeast Asia type alpha-thalassemia heterozygosity and 43 normal gene type resulted from Gap-PCR, and 33 beta-thalassemia heterozygosity and 36 normal gene type diagnosed by reverse dot blot (RDB). Cutoff value of Hb-A2 in finding of alpha-thalassemia and beta-thalassemia was present.The sensitivity, specificity, positive and negative likelihood ratio, and receiver operator characteristic curve (ROC) were used to evaluate the significance of Hb-A2 level in screening thalassemia in adults. Results The borderline values between normal and pathological range of Hb-A2 were ≤2.6% in alpha-thalassemia and ≥3.8% available in beta-thalassemia. Sensitivity and specificity were 59% and 54% in alpha-thalassemia, 76% and 83% in beta-thalassemia, respectively. ROC showed that the area covered beneath the curve of beta-thalassemia was more than that of alpha-thalassemia. Conclusions The cutoff between normal and pathological range of Hb-A2 is ≤2.3% in alpha-thalassemia and ≥3.8% in beta-thalassemia. Hb-A2 detection has higher sensitive and specific in beta-thalassemia diagnosis, but lower in alpha-thalassemia diagnosis.