Background: Prognosis of the preterm infant depends more on gestational age than birth weight, but clinical assessment of gestational age of preterm infant is inaccurate. Objective: To assess whether cerebellar vermis diameter is accurate in predicting gestational age in premature infants. Materials and Methods: We obtained suitable midline sagittal images of the cerebellar vermis at cranial sonography, performed via the anterior fontanelle, in the 40 preterm infants, from the regional neonatal intensive care unit whose gestational ages were assessed. We measured the cerebellar vermis diameter on the hard copy image provided the margins of the vermis were clearly visible. Vermis diameter was measured from the base of the fourth ventricle to the junction offolium and tuber vermis. We used regression equations for estimating gestational age using combination of birth weight and vermis diameter. Results: Measurement of cerebellar vermis diameter by cranial sonography allowed predication of gestational age to +/-1.3 weeks using a 95% prediction interval. Conclusion: Assessment of gestational age by measuring cerebellar vermis diameter using cranial sonography proved to be accurate and simple method for assessing gestational age. It is more objective, easier, less time consuming and less distressing to the baby than the New Ballard Score.