This study was undertaken to establish a noninvasive prenatal genetic diagnostic method for trisomy 21 using the fetal nRBCs that is rarely present in maternal circulation. Peripheral venous blood samples were collected from 30 women with an advanced maternal age, abnormal triple marker test results, or abnormal ultrasound findings such as an increased nuchal translucency. The blood samples were treated with heparin. The triple density gradient centrifugation, and MACS using CD45 and CD71 were used to isolate the fetal cells. FISH analysis using probe 21 was performed with GPA-immunostaining. The study population consisted of 30 patients from 13 to 25 weeks of gestation, and nRBCs were separated in all cases. In GPA-immuno FISH analysis using probe 21, 3 cases of trisomy 21 were diagnosed and these results were confirmed by the amniocentesis. In conclusion, a prenatal diagnosis of trisomy 21 through GPA-immuno fluorescence in situ hybridization (FISH) analysis using separated fetal nRBCs is a useful, innovative, accurate, rapid and non-invasive diagnostic method.