Abstract: Biological soil crust (biocrust) is a compact complex layer of soil, which has photosynthetic activity and is one of the factors affecting the CO₂ flux of soilatmosphere interface. In this paper, the soil CO₂ flux under the effects of biocrust at different succession stages on the re-vegetated grassland in the hilly region of Loess Plateau was measured by a modified LI-8100 automated CO₂ flux system. Under light condition, the soil CO₂ flux under effects of cyanobacteria crust and moss crust was significantly decreased by 92% and 305%, respectively, as compared with the flux without the effects of the biocrusts. The decrement of the soil CO₂ flux by the biocrusts was related to the biocrusts components and their biomass. Under the effects of dark colored cyanobacteria crust and moss crust, the soil CO₂ flux was decreased by 141% and 484%, respectively, as compared with that in bare land. The diurnal curve of soil CO₂ flux under effects of biocrusts presented a trend of ‘droprisedrop’, with the maximum carbon uptake under effects of cyanobacteria crust and moss crust being 0.13and -1.02 μmol CO₂·m^-2·s^-1 and occurred at about 8:00 and 9:00 am, respectively, while that in bare land was unimodal. In a day (24 h), the total CO₂ flux under effects of cyanobacteria crust was increased by 7.7%, while that under effects of moss crust was decreased by 29.6%, as compared with the total CO₂ flux in bare land. This study suggested that in the hilly region of Loess Plateau, biocrust had significant effects on soil CO₂ flux, which should be taken into consideration when assessing the carbon budget of the ‘Grain for Green’ eco-project.