Purpose: Between 2017 and 2018, the prevalence rate of vitamin D deficiency (VDD) among children and adolescents in Korea (single institution) was 68.4 percent (59.6 percent for males and 72.5 percent for females). However, effective vitamin D supplements are not consistent in literature. We tried to find out about the dosage and the duration of the administration. Methods: The study was conducted on 2,770 children aged 0-18 who tested serum vitamin D concentrations for outpatients and inpatients at our hospitals from August 2017 to July 2019. One group was treated with maintenance doses and the other group was treated with maintenance doses after oral vitamin D 2000 IU/d for six weeks. The maintenance dose was 400 IU/d before puberty and 1000 IU/d after puberty. Results: There was a significant correlation between serum 25(OH) vitamin D concentration and gender, age, season, weight SDS and BMI SDS (p=0.000, p=0.000, p=0.000, p=0.000, p=0.002, respectively). After 6 months of oral Vitamin D treatment, serum 25 (OH) Vitamin D concentration was increased in both groups (p=0.000, p=0.000, respectively). In a group treated with maintenance doses after oral vitamin D 2000 IU/d for six weeks, it was found a higher rate of change to vitamin D sufficiency (p=0.000). Conclusions: The prevalence rate of VDD increases in female, older age, overweight and winter. The serum 25 (OH) vitamin D concentration increased in both groups after 6 months of treatment. In VDD children, It seemed appropriate to take an oral Vitamin D 2000 IU/day for 6 weeks before maintenance treatment according to the current guideline treatment.