Abstract: A stabilizing factor of NR activity was isolated from tomato leaf homogenate by (NH4)2SO4 precipitation, boiling water treatment, Sephadex G-75 column and DEAE 52 column and dialysis against water. This factor appeared as band on polyacryl amide gel electrophoresis. This factor protected NR activity of tomato leaves from inactivation by temperature. It delayed the inactivation of N-R activity in wheat seedlings. when mixed with stabilizing factor and stored at 0 ℃ for nine days NR from wheat seedlings still kept its activity; without stabilizing factor, its activity was lost completely. Stabilizing factors both treated in boiling water in the process of isolation and isolated below 4 ℃ could stabilize and increase NR activity of tomato leaves. This factor existed in two forms, one free and one combined with some protein. It could be separated from the protein by heating during extraction. The stabilizing factor is different from FAD, haem, and proline.