Objective: The diabetic patients may develop electrolyte imbalance. This study aims to determine an inverse relationship between serum sodium (Na+) and potassium (K+) levels in diabetic patients. Methods: This cross-sectional was conducted in KIST Medical College and Teaching Hospital, Lalitpur, Nepal over a period of 2 months (June to July 2016). Total of 135 patients (65 male and 70 female) diagnosed with type 1 and type 2 diabetes mellitus of age ranged from 14 to 85 years were included in this study. Fasting blood glucose (FPG) level was analyzed with semiautomatic analyzer- humalyzer 3000 by Glucose oxidase/Peroxidase method and Na+ and K+ levels were analyzed with ion selective electrode-nova electrolyte. The relationship among serum Na+ level, serum K+ levels and FPG levels were determined by linear regression and ANOVA with software SPSS version 20. A 95% confidence interval was used. Results: Serum Na+ level was insignificantly negatively correlated (r=- 0.091, p=0.296) with FPG level while a positive correlation of serum K+ level (r=0.235, p=0.006) was seen with FPG level and an inverse relation between serum Na+ and K+ was found. Age showed insignificant negative correlation with serum Na+ (r= -0.203, p=0.018), insignificant positive correlation with K+ (r=0.067, p=0.443) and insignificant negative correlation with FPG (r= - 0.045, p=0.608). Conclusion: Hyperglycemia disrupts the balance of serum Na+ and K+ in uncontrolled diabetes mellitus. This study showed hyponatraemia and hyperkalaemia with increased hyperglycemia i.e. there is inverse relation between serum Na+ and K+ levels and it is dependent plasma glucose level.