Introduction: Type-1 Diabetes Mellitus (T1DM) is a common chronic disease of childhood which presents with acute, sometimes life-threatening, symptomatic hyperglycemia.T1DM is at least a great risk factor for cardiovascular mortality as Type-II Diabetes Mellitus. Thus, the detection and treatment of these risk factors in T1DM are warranted. Some data support the idea that atherogenic abnormalities of diabetic patients with hypomagnesaemia may start in childhood, and suggest that hypomagnesaemia diabetic patients might be at higher risk for premature cardiovascular disease development. Aim of the Study: To assess the levels of serum Mg and their relations to the development of cardiovascular complications in children with T1DM. Subjects and Methods: This study was conducted upon 80 children with (T1DM) ,40 children newly diagnosed with T1D and 40 children with long duration(>5years) ,they were randomly selected from the pediatric diabetes & endocrinology outpatients’ Clinic, Maternity and Children Minia University Hospital, Minia ,Egypt. Another 40 apparently healthy, age and sex matched to the diabetic group were taken as a control group. The study was conducted during the period from December 2020 to December 2021.All children were subjected to the following: history taking, clinical examination and laboratory investigations including; Fasting and plasma glucose two hours postprandial, Glycosylated Hemoglobin (HbA1c%), Lipid profile :serum Cholesterol, Triglycerides, high density lipoprotein, low density lipoprotein. Serum Magnesium , Potassium and Calcium levels. Doppler ultrasonography for Carotid intima media thickness (IMT) before and after magnesium supplementation for 6 months.