Introduction: Gastroenteritis (GE) is one of the most common pediatric diseases. Hyponatremia commonly occurs by administering hypotonic fluids to GE and hospitalized children. Yet, there is no consensus on the ideal method of treatment. Objectives: we aimed to assess suitable intravenous (IV) fluid for preventing dysnatremia in children with GE. Patients and Methods: This is a double blind randomized clinical trial, which was conducted on infants of 6 months up to 14 years children with GE. Children were randomly assigned in 2 different groups. Group A; received 20 cc/kg 0.9% isotonic saline as a bolus, and 0.45% hypotonic saline as sum of maintenance fluid and volume deficit. Group B was treated with the same bolus and 0.9% isotonic saline with 20 mEq/L KCl as sum of maintenance fluid and volume deficit. Blood and urine samples were taken at admission, 4 and 24 hours. Data were analyzed by independent t test, Mann-Whitney U test, Friedmann test, chi-square and 2-tailed repeated measurements by SPSS version 19. Results: Baseline hyponatremia and isonatremia were detected in 24 (31.5%) and 51 (67.1%) patients, respectively. Mean level of sodium at T0, T4 and T 24 mentioned no significant difference between groups. No hypernatremia was noted by administering isotonic saline. Results showed that 4 and 24 hours after administration isotonic saline, the mean plasma sodium differed significantly in baseline hyponatremic patients. However, no significant difference was noted after 4 and 24 hours in group A. Conclusion: According to the considerable effect of isotonic saline on hyponatremic patients, it seems that administering isotonic fluids regardless of the types of dysnatremia can be recommended to lessen clinicians’ conflicting decision-making in selecting an appropriate fluid.