Along the line of a nonproliferation policy for nuclear materials, the development of a process for the co-conversion of Pu-U nitrate mixed solutions to mixed-oxide (MOX) powder has been carried out, and through the bench-scale experimental work performed at the Power Reactor and Nuclear Fuel Development Corporation, a new co-conversion process using a microwave heating method has been successfully developed. Using the bench-scale test unit with a 2 kg of MOX/day capacity, designed on the basis of the results of mock-up tests, the feasibility of this process has been demonstrated with good results. In this test unit installed in the Plutonium Fuel Fabrication Facility, a fixed volume (∼7.2 ℓ )of Pu-U nitrate mixed solution [∼270 g (Pu + U)/ℓ] is fed into a denitration vessel placed in a fixed position on the denitration apparatus. When microwave power (2450 MHz; maximum 16 kW) is applied to the mixed solution, the solution is directly heated and changed to PUO2−UO3 MOX through the denitration reaction. This PUO2−UO3 MOX is then scraped off the denitration vessel and transferred so the subsequent processes of calcination-reduction and pulverization can be performed. In these processes, it is changed to PuO2−UO2 MOX powder, which is suitable for pellet fabrication. Through the basic experiments with the test unit, some important data were successfully obtained concerning the feasibility of the test unit, powder characteristics of the product, and impurity pickups during the denitration process. At present, using this unit, plutonium nitrate solution transported from the Tokai Reprocessing Plant is being converted to MOX powder which has a Pu/U ratio = 1. Stable operation continues and the products have been used without any difficulty as the raw material for advanced test reactor “FUGEN” reloading fuel fabrication. Judging from the result of these experiments, the co-conversion process using a microwave heating method has been found to have many excellent advantages, such as good powder characteristics of the product, simplicity of the process, good homogeneity of the plutonium and uranium oxide, minimum liquid waste, no impurity pickup during denitration, no possibility of changing the Pu/U ratio, and small “material unaccounted for” through the process. This co-conversion process has been adapted to the main process of the co-conversion facility with a 10 kg of MOX/day capacity, which is now under construction as the end process of the Tokai Reprocessing Plant.