Thalassemia is a genetic condition that can result in long and expensive treatments, and severe thalassemia may lead to death if left untreated. Couples contributing two genes for thalassemia place their children at particular risk for severe thalassemia. Gene frequency of thalassemia varies in Vietnam, but presents remarkably high levels among some ethnic minority groups. Limited information about thalassemia frequency makes prevention and control of thalassemia difficult. This study aimed to determine gene frequency of certain types of thalassemia among 390 women of reproductive age of the Ta-Oi ethnic minority. Hemoglobin and DNA analyses were carried out to diagnose thalassemia and hemoglobinopathies. Of the total participants, 56.1% (95% CI = 51.1–61.1) carried thalassemia genes. A remarkably high frequency of hemoglobin Constant Spring (Hb CS) of 23.8% (95% CI = 19.7–28.4) was noted. The frequency of α+-thalassemia (−3.7 kb deletion) was 26.4% (95% CI = 22.1–31.1), while hemoglobin E (Hb E) and hemoglobin Paksé (Hb Ps) were identified at frequencies of 14.6 (95% CI = 11.2–18.5) and 2.6% (95% CI = 1.4–5.0), respectively. Further analysis of α-globin gene haplotype revealed the same Hb CS haplotype (+ − M + + −) as of the Co-Tu minority, a neighboring minority of the Ta-Oi, indicating that these two minorities may share the same ancestors. This information will be helpful for further studies in population genetics, as well as the development prevention and control program in the region.