As the first step on utilization of X‐ray microscope to tumor diagnosis at sub‐cellular level, we have started investigating intracellular localization of heavy metal marker in cancer cells. We calculated the image
contrast and photon density for platinum and gold of 100 nm thickness in the protein at various X‐ray energies. At hard X‐ray region, phase contrast gave effective contrast to image heavy metal marker localization in the cancer cell of 10μm thickness. Minimum incident photons on the specimen were calculated as about 1011 photons/mm2. Combined with a suitable tumor marker and a high brilliant X‐ray compact source. X‐ray microscopy has capability to be in use as a tumor diagnosis tool with high special resolution.