We have investigated chemical forms of molybdenum ion in nitric acid solution, using liquid-phase X-ray absorption fine structure, ultraviolet-visible absorption spectroscopy and first-principles calculations, from a viewpoint of disposal of high-level radioactive nuclear liquid wastes. The experimental and theoretical results indicated that Mo is a hexavalent ion and forms a hexa-coordination binuclear-structured complex in the 2 M nitic acid solution. The predominant chemical species of Mo complexes in the 2 M nitric acid solution (which is used for HLLW) are assigned to be [Mo2O5(H2O)(6)](2+) and [Mo2O4(OH)(H2O)(6)](3+). These species may play a crucial role of forming so-called yellow-phase in vitrified objects.