The operation condition far improvement of reduction degradation index (RDI) at 550℃ was investigated by analyzing the daily operation reports of minter plant during the period of 1983 Jan.-Dec. The experimental test on the base of the adopted factors was performed in a pot of 240㎜Φwith bed height 390㎜ under the condition of the same basicity and coke ratio. The applied test based on th findings of the important factors was conducted at the same sinter plant. The results obtained may be summarized as follows: For the improvement of RDI, followings should be carefully controlled 1) Magnetite ratio of raw mix: under 16% 2) Slag volume ratio : about 20.0% 3) TiO₂ ratio : under 0.21% 4) Bed height should be controlled with the change of burnt lime addition ratio.