The aim of the present study was to develop regression models based on ultra-sonographic measurements of fetal eyes (orbital diameter (OD) and eyes volume (EV)) to determine mare gestational age (fetal age) in Arab-Barb breed and to compare the obtained formula with similar formulas used in practice. Twenty-two Arab-Barb mares were included in this study. For each mare, an average of 6 ultrasound examinations per month of gestation, from day 90 of pregnancy to foaling were performed. The parameters examined, OD and EV, were used to establish regression models to determine gestational age of mares. The results led to establish linear regressions of the two parameters, EV and OD. The derived gestational age (GA) prediction formulas were: GA = 0,0097EV + 93,887; GA = 8.687OD + 0.6598 (GA in day; EV in mm3; OD in mm). The coefficients of correlation (R) between EV (R=0.96), OD (R=0.93) and mare gestational age were highly significant (P<0.001) indicating that the two eye measurements can be used to determine accurately mare gestational age after 90 days of pregnancy in Arab-Barb breed. In addition, the formula established in the present work from the OD is more accurate in determining the GA in the Arab-Barb compared to that predefined for English Thoroughbred. This comparison reveals that the growth of fetal orbit of Arab-Barb breed is slower than that of the English Thoroughbred.