Holographic optical elements (HOEs) are optical components that utilize the diffraction principle. For decades, due to their versatility, high efficiency, and low cost, people have created and utilized HOEs in holographic displays, imaging, laser beam shaping, and certain scanning technologies. One of the widely used applications of HOEs is diffraction grating. Volume holographic gratings offer several advantages over grating films, such as higher diffraction efficiency and a wider spectral range. However, volume holographic gratings have traditionally been evaluated using indirect methods, such as diffraction efficiency or 2D phase imaging. In this poster, we visualize the three-dimensional structures of volume holographic gratings using optical diffraction tomography to evaluate the structure. These images reveal uniformity and defects in the samples.