Radial fossa is an indentation above the capitulum and is found on the anterior surface of humerus. The head of radius fills the radial fossa at the time of extreme flexion of the elbow joint. At the lower end of humerus, the capitulum joins together with the upper curved in surface of the head of radius. Radial fossa is the least distinct fossa of all three fossae.
Radial fossa is found in lower end of the humerus on the anterior side just above the capitulum. See the diagram above to know the exact position.