An aberrant axillary artery running deep to the brachial plexus during its course was observed. The brachial plexus in this case was formed by the fourth cervical nerve to the first thoracic nerve and the radial nerve received a small nerve bundle (accessory radial nerve root) from the posterior aspect of the lower trunk. The axillary artery passed between the lateral and the medial cords of the brachial plexus, the same as for an ordinary axillary artery. In addition, it passed between their posterior divisions, forming the posterior cord, and further ran between the radial nerve root and the accessory radial nerve root from deep to superficial. This axillary artery was recognized as the deep axillary artery, the same as the axillary artery we have reported previously. It was thought that the branches to the subscapularis and serratus anterior muscles, nutrient branch to the radial nerve root and the subscapular artery played important roles in its formation.