Background: The exact determination of gestational age is so important in management of the antepartum care, and for adequate planning of proper intervention or therapy. Tanscerebellar diameter has great advantages in prediction of gestational age in cases of uncertain dates or in suspected intrauterine growth retardation. There are minimal data available about the relationship between tanscerebellar diameter and biparietal diameter in third trimester of pregnancy. Objective: The aim of the present study was to assess the accuracy of transcerebellar diameter (TCD) measurement in estimation of the gestational age during the third trimester compared to the current fetal biometric measurements including femur length and biparietal diameter. Patient and Method: The study included 500 pregnant women with sure and reliable dates fulfilling the inclusion criteria at the Department of Obstetrics and Gynecology, Al-Azhar (Bab Alsheria ) university Hospital, (inpatient and outpatient) from May 2018 till November 2018. The entire subjects were in the third trimester of pregnancy seen at 31 – 36 weeks, the transcerebellar diameter, the biparietal diameter and femur length were measured for determination of gestational age. Results: The results showed that the transcerebellar diameter (TCD) is more accurate than the biparietal diameter (BPD). There were insignificant statistical difference between transcerebellar diameter (TCD) and femur length (FL) for determination of gestational age in the third trimester whereas there was a significant difference between the transcerebellar diameter (TCD) and the biparietal diameter (BPD) for determination of gestational age in the third trimester. All those data were compared to the last menstrual period. Conclusion: Transcerebellar diameter is more reliable method of gestational age determination in third trimester of pregnancy than biparietal diameter. Transcerebellar diameter (TCD) and femur length (FL) can be used as a tool to assist in the assessment of gestational age in third trimester