Introduction: Glucose Homeostasis can potentially be affected by Vitamin D.  Vitamin D deficiency has been identified as a global problem with an estimated one billion people worldwide suffering from vitamin D deficiency or insufficiency. Vitamin D deficiency (VDD) is a global health issue and is on the rise in Pakistani pediatric population. Type 1 diabetes (T1D) also negatively impacts bone health and is associated with a modest reduction in bone mineral density (BMD) and strength (7,8) Given the negative impact of vitamin D inadequacy and T1D on bone health, Adolescent with both conditions have multiple risk factors for increased skeletal fragility. In this study we evaluated glycemic changes after vitamin D3 supplement in patients with type I diabetes mellitus and vitamin D deficiency. Objective: To determine the mean glycemic changes after vitamin D3 supplement in children with type I diabetes mellitus and vitamin D deficiency presenting at a tertiary care hospital in Karachi. Methodology: A quasi experimental study design was conducted on 40 children who were presenting in OPD of NICH Karachi for six months. All patients were subjected to demographics and detailed history for Type 1 Diabetes and Vitamin D deficiency. Immunoturbidometry for HbA1C and ELISA for 25OHD was performed. Vitamin D3 supplement was advised as per operational definition. Insulin dose was recorded by parents for three months. 25OHD and HbA1C levels were measured following Vitamin D3 supplements. Statistical analysis was performed to analyze the significance. Results: We observed a significant difference in mean HbA1c level at pre-treatment and post treatment was 9.89±2.04 and 8.42±1.87 respectively with Vitamin D3 supplements, with p-value 0.001. Conclusion: Vitamin D3 supplement improves HbA1C in patients with type I diabetes and vitamin D deficiency.