The aim of this study was to study the bioeffects of diagnostic dynamic 3-dimensional ultrasound (4D) on ultrastructure of cerebral cells of fetal mice in late pregnancy. Thirty pregnant mice carrying 18th embryonic day fetuses were randomly allocated into 6 groups, namely, control group, sham-exposed group, 5 minute–exposed group, 10 minute–exposed group, 20 minute–exposed group, and 30 minute–exposed groups (5 in each group). In exposure groups, mice were put under the dynamic 3D ultrasound system's probe for 5 to 30 minutes. Mice in sham-exposed group did not receive ultrasound wave. At 24th hour after birth, 10 pups of each group were randomly selected (2 in each litter) and euthanized by decapitation, and the brains were immediately removed. Right parietal lobes were taken as specimen. The specimens were firstly fixed with glutaraldehyde and secondly with osmic acid, then sections were made and observed under the transmission electron microscope. There were no obvious abnormal ultrastructure changes in control group, sham-exposed group, and 5 minute–exposed group under transmission electron microscope. Ten minute–exposed group showed some enlarged mitochondria, broken crista, vacuolated endoplasmic resticulums, and a few apoptosis cells. More abnormal organelles and apoptosis cells were observed in 20 minute–exposed and 30 minute–exposed groups (P < 0.05). Dynamic 3D (4D) ultrasound exposure for more than 10 minutes may result in abnormal neuronal ultrastructure changes and apoptosis cells in fetal mouse cerebrum.