Due to complex material and nonuniform characteristics of reinforced concrete members, practical obstacles exist when conducting nondestructive testing for determination of the position of holes in concrete. This paper discusses how to use gamma-rays to penetrate concrete and project images of internal structures and holes/discontinuities on film. By analyzing the geometry of projected images on the film, the size and position of holes in concrete members can be obtained. Referring to gamma-ray exposure curves on reinforced concrete structures, this paper also establishes a relation of gamma-ray sources and thicknesses of concrete to calculate the gamma-ray exposure time. It will help inspectors and engineers to effectively estimate the quality of reinforced concrete structures.