This study was performed to establish a new parameter for estimating gestational age and predicting parturition day by ultrasonographic measurement of deep portion of telencephalic vesicle (DPTV) diameter in small dogs. Fetal head diameter (HD) and DPTV diameter were measured in 15 pregnant Pekingese bitches, from Day 15 to the parturition day, and evaluated the correlation between gestational age. HD was measured from day 29 of pregnancy to parturition day and increased from 4.9 ± 2 mm to 25.5 ± 0.7 mm. Especially, from day 38 of pregnancy to parturition day, HD uniformly increased about 0.6 mm per day and was significantly and linearly relative to gestational age during this period (r2>0.99). DPTV diameter was measured from day 35 to day 60 of pregnancy and increased from 3.2 ± 0.9 mm to 11.5 ± 0.7 mm. Especially from day 38 to day 60 of pregnancy, DPTV diameter uniformly increased about 1 mm per 3 days and was significantly and linearly relative to gestational age during this period (r2>0.99). In conclusion, DPTV diameter could to be a useful parameter for the estimation of gestational age and the prediction of parturition day when used alone or in combination with HD during the second half of pregnancy.