Internal jugular phlebectasia (IJP) is a fusiform dilatation of the internal jugular vein (IJV), usually presented as a neck mass in children. Accurate diagnosis from carefully directed history, physical examination, and radiological study could result in lifesaving therapy. We performed our study to suggest possible clinical diagnostic criteria for IJP in Korean children. We reviewed three cases of IJP (patients group) and compared the diameter of the internal jugular phlebectasias with diameters of IJVs in ten normal children (control group) using ultrasonography (USG). There were no significant differences in the range of diameters in the resting state between the two groups. The diameters on the right side, compared with those on the left side, showed no statistical significance (p > 0.05). Te range of expanding diameter and average expanding ratios (resting state to Valsalva maneuver × 100%) showed a statistical difference between the two groups (p < 0.05).