Abstract: Diuron (DCMU) blocks the electron transporting from QA to QB in the thylakoid membrane. Delayed fluorescence(DF) in plants is an intrinsic label of the efficiency of charge separation at P680 in photosystemⅡ (PSⅡ). In this paper, with spinach as a testing material, we studied the effects of DCMU on photosynthesis of plant from the characteristics of DF decay kinetic curve and its total intensity. The fast phase of DF decay curve of leaves obviously rise after DCMU treatment, while the relative slow phase declined. With the different concentration of DCMU treatment, the intensity of the DF and net photosynthesis rate (Pn) behaved excellent correlation. All the results suggested that DF is an ideal marker to characterize the Pn of plant after DCMU treatment.