Management of children with acute gastroenteritis is based upon dehydration estimation. There is no clinical or paraclinical tool which exactly estimates the dehydration degree. Recently ultrasonographic parameters as inferior vena cava (IVC) diameter and aorta (AO) have been used in some studies for this purpose. This study aims to evaluate the efficacy of ultrasound in detecting mild and moderate degrees of dehydration in children. The study was performed in the emergency department of Dr. Sheikh’s Children Hospital, Mashhad, Iran. Children with mild to moderate degrees of dehydration according to World health Organization (WHO) clinical scale were enrolled. Their inferior vena cava diameters, aorta and IVC/AO ratio were measured before and after fluid therapy using ultrasound. Ultrasound was performed by two pediatric sonographers. 36 patients (mean age of 16.94±11.02 months) entered the study. 11 patients had mild and 25 moderate dehydration according to WHO clinical scale. All 11 patients with mild dehydration received oral rehydration. 13 patients in the moderate dehydration group received intravenous rehydration because of oral intolerance to fluids and recurrent vomiting. IVC diameter and IVC/AO ratio after fluid therapy in children with both mild and moderate dehydration degrees was significantly greater (P<0.001). However, we did not observe any significant difference in aorta diameter before and after fluid therapy. Using Receiver Operating Characteristic (ROC) curve, the proper cut-off point of IVC/AO ratio to differentiate patients with moderate dehydration from mild dehydration is equal to 0.782 with sensitivity and specificity equal to 88% and 45.45% respectively. Further, the area under the ROC curve for this cut-off is equal to 0.569. In conclusion, ultrasonography cannot differentiate between mild and moderate dehydration degrees, but studies with larger population of patients should be performed.