Magnesium depletion has a negative impact on glucose homeostasis and insulin sensitivity in diabetic patients. Low plasma magnesium concentration is a highly specific indicator of poor magnesium status. This case control study was conducted in Mymensingh Medical College Hospital, Bangladesh, from May 2011 to April 2012. This study was done to assess the serum magnesium level in diabetic and non-diabetic patients and to find out the association between serum magnesium, blood glucose and duration of diabetes mellitus. Serum magnesium concentrations were determined in 50 diabetics and 50 age and sex matched non-diabetic patients or attendants for controls. Among the case and control group 32 were male and 18 were female. Mean age of control group was 54.42±2.33 and study group was 55.78±2.43 years. Duration of diabetes of case group in which 50% of patients have diabetes for up to 4 years, 18% of patients have diabetes for 5-9 years, 5% of patients have for 10-14 years and 2% of patients have diabetes for 15 years and above. Serum magnesium concentrations below the normal reference range for study group was 44% and control group was 6%. The mean serum magnesium level of study group was 0.70±0.01mmol/L and that of control group was 0.83±0.02mmol/L. So, the difference in serum magnesium level between two groups were statistically highly significant (p = 0.001). Correlation between FBS and SML of study group, which is negative (r = - 0.182). There is opposite relation among the characteristics as r is negative. And also correlation between 2HAFB and SML of study group, which is negative (r = - 0.06). There is opposite relation among the characteristics as r is negative.