Transmutation of 129I to 130I by (n, γ) reaction and isotope dilution with stable iodine were discussed. The transmutation in LWR is calculated by supposing that targets irradiated for 25 years are substituted with new targets. The result showed that initial amount of 129I will be reduced to 11.5%. In order to lower dose equivalent of general public than 0.1mSv/y, 400 times isotope dilution of 129I is required. The feasibility of the isotope dilution was discussed at dissolution process of spent fuel, conditioning process of the waste and disposal conditions.