Background: The prevalence of subclinical hypothyroidism(SCH) varies with population, age, sex. Age is one of the most significant risk factors according to several epidemiologic studies. The prevalence of SCH and mean serum TSH are known to increase with aging and more common in women in adult population. Purpose: This study aimed to evaluate the national prevalence of SCH in children and adolescents from South Korean population. We examined the prevalence of SCH and the association of age with it, free T4 and TSH. Methods: We analyzed nation-wide cross-sectional data from the Korean National Health and Nutrition Examination Survey VI(2013 to 2015). The data included blood testing for thyroid function and anti-thyroid peroxidase antibody(TPO Ab). We selected individuals aged from 10 to 18 who did not report thyroid disease or receiving thyroid-related treatment(n=1,010). SCH was defined as TSH(mIU/L) ＞4.5 and 0.7≤fT4 (ng/dL)≤2.0. Results: The prevalence of SCH was 14.1% (142/1010). Unlike from the result of other studies, there was no statistically significant difference between male(15.0%, 79/142) and female(13.0%, 63/142). The prevalence of SCH decreased with age in the male subject(odd ratio 0.873, p-value 0.028) but there was no significant change in female. And TSH decreased with aging in both sex groups(p-value 0.000) and free T4 increased with age only in male subject(p-value 0.011). In addition, the prevalence of SCH in men increased as total cholesterol increased by 10mg/dl(odd ratio 1.111, p-value 0.026). Other factors known to be associated with SCH prevalence in adult did not show any significant results in children and adolescents. Those factors included TPO Ab, BMI, Urine iodine and cigarette smoking. Conclusion: This study provides some baseline information for understanding patterns of thyroid dysfunction in children and adolescents. Decreasing prevalence of SCH with aging in male might result from the change of TSH to lower concentration with increasing age.