The objective of this work was to observe the side effects on the peripheral blood treated with 131I in children and adolescents with differentiated thyroid carcinoma (DTC). The levels of white blood cells (WBC), neutrophil, HGB (hemoglobin), RBC (red blood cells) and the PLT (platelet) were measured by automatic hemocyte analyzer in 31 young enrolled patients (< or = 20 years old) before 131I therapy, 1 month and 6 months after 131I therapy, and at the end of follow up. The results of 131I treatment at the before and after the treatment were compared. The level of PLT decreased obviously (P < 0.01), WBC, RBC and HGB decreased (P < 0.05), neutrophil increased slightly (P > 0.05) 1 month after 131I therapy. The level of WBC, neutrophil, RBC, HGB and PLT were near to the level of before treatment after 6 months (P > 0.05) of 131I therapy. At the end of follow up, the components of the peripheral blood in the high dose group (7.4GBq-11.1GBq) were not significantly different from those at the beginning. The results showed that, there was only a transient impact on the peripheral blood by the 131I therapy in the children and adolescents with differentiated thyroid carcinoma.