Background: Desmoid tumor (DT) is rare and challenging, often affects head neck (HN) region in children, and its appropriate treatments are under discussed. This study aimed to retrospectively evaluate long-term effectiveness and safety of ¹²⁵I seed brachytherapy for pediatric DT in HN. Procedure: Seven pediatric patients with median age of 3 years old suffered from DT in HN treated with ¹²⁵I brachytherapy from January 2008 to June 2018 were included. Among which, 5 underwent sole brachytherapy and the others combined with surgery under a prescription doses ranging from 10 000 cGy to 12 000 cGy. The rate of local control (LC), complete response (CR) and partial response (PR) were calculated after evaluation by radiological and pathological means. The radiation-associated toxicities were also evaluated Results: The LC rate was 7/7 during the follow-up time ranging from 43 to 135 months and with a mean of 57 months. No recurrent lesion was found in the patients receiving surgery combined with brachytherapy. In patients treated with sole brachytherapy, the radiological PR rate and CR rate were 4/5 and 1/5, respectively. In those reaching radiological PR, 3/4 were pathological CR. Slight acute radiation-associated toxicities were observed in all patients, and no late or severe acute toxicity was observed. Conclusion: ¹²⁵I brachytherapy is effective and safe in the management of pediatric DT in HN as sole modality or combined with surgery in long term.