Background: Type 1 diabetes mellitus (T1DM) is highly prevalent endocrinal disorder. with incidence of around 8/100,000 per year in Egyptian children under the age of 15 years. Evidence indicate that Magnesium (Mg) plays an important role in prevalence of T1DM. and its complications. Thus we aimed to find out the effect of T1DM. on serum Mg level. Methods: We included 50 Egyptian children with type 1diabetes at Pediatric outpatient clinic, Al-Hussein University Hospital and El-Sahel Teaching Hospital, during the period from May, 2017 to December, 2017, and 25 age- and sex-matched healthy individuals in a case-control study. We measured Serum magnesium, HbA1c, random blood suger, serum urea, and creatinine level and CBC. Results: Diabetic children had significantly lower serum magnesium level compared to control children (1.91 ± 0.22 mg/dL in diabetic children versus 2.08 ± 0.19 mg/dL in control children). Hypomagnesemia was detected in 26% of diabetic children compared to 8% of control children. Significant negative correlation between serum Mg and HbA1c was detected. (p₌ 0.000 r₌ -0.703). also negative correlations between Mg and each of age and duration of DM. (p₌ 0.016; r₌ -0.339); (p= 0.000; r₌ -0.339) respectively. Conclusion: patients with T1DM more liable to develop hypomagnesemia specially with poor glycemic control. Recommendations: Proper glycemic control and regular monitoring of serum magnesium in children with T1DM.