We aimed to make first demonstration of the course and branching pattern of deep branch of the radial nerve (DBRN) in relation to the supinator muscle in the forearms of fetuses and make morphological measurements in both fetuses and adults. We carried out dissection on 80 radial nerves from 20 adult cadavers (16 males and 4 females), ranging from 30 to 56 years old and 20 formalin-fixed, spontaneously aborted fetuses (10 males and 10 females) without detectable malformations, ranging from 20 to 37 weeks of gestation. There were three different branching patterns of the DBRN in fetuses. It divided its terminal branches at entrance of the supinator in 2 (5%), after entry of the supinator in 4 (10%) and after the supinator in 34 (85%) forearms. The course was bilaterally same in fetuses. Dividing pattern at the entrance of the supinator was not seen in adults. Only 4 (10%) were seen at after entering to the supinator and 36 (90%) were seen at after the supinator. We saw in fetuses neither fibrous band deep from the extensor carpi radialis longus nor upper border of the supinator. We assume that these fibrous structures appear after birth, perhaps favored by physical exercise, such as repeated pronation–supination movements. We think that it is one of the reasons why DBRN compression has never been reported in children. This is the first fetal study about the course and branching pattern of the DBRN and there is a constant ratio (mean 4.3) between the measurements of the fetuses and adults.