The aim of this study was to assess the relationship of vitamin D level and supplement with glycated hemoglobin level in children with T1DM attending a tertiary institution in Oman. In addition, we described the trend of vit D deficiency among included patients. For this, a prospective cohort study was conducted. Data was collected for total of 131 pediatric patients with T1DM. Vitamin D level was measured at the baseline (2014) and after one year. Their glycated hemoglobin was collected at the baseline of the study and at three subsequent time points from the baseline within the 1-year period. Subjects were categorized according to their vitamin D status at baseline to; sufficient, insufficient and deficient. Patients received vitamin D supplementation according to their vitamin D status for one year. At the end, the mean glycated hemoglobin for a 9 months period was compared. 59 patients (45%) were vitamin D sufficient, 46 (35%) were vitamin D insufficient and 26 (20%) were vitamin D deficient. Their baseline HbA1c was 9.6%, 9.6% and 8.7% for the three groups respectively. There was no significant difference in the subsequent mean HbA1c in the three groups. HbA1c of insufficient group changed insignificantly from 9.5% to 9.6%, while HbA1c of deficient group changed insignificantly from 8.7% to 9.3%. Vit D inadequacy dominates the trend of this vitamin in Omani children with type T1DM. It seems that neither the level, nor the supplementation of vitamin D supplementation has an effect on glycemic control.