Abstract:In nitrate reductase extracting solution, we added excess amount of polyclar AT, and decreased the pH to 7.0, correspondently increased the pH‘s of the other substrates, thus made the final pH of the reaction system in the range of 7.6—7.8. It was shown that with the treatment of Zn(AC)2 and PMS to get rid of the excess NADH, factors interfering in nitrite coloration, could be eliminated from this reaction. The inductive conditions of NR and some enzymological characteristics of it have also been studied. Thus a stable and feasible method for the measurement of nitrate reductase activity in vitro in populus leaves has been established.