Abstract: Chlorpyrifos (O,O-diethyl-O-3,5,6-trichloro-2-pyridinyl) phosphorothioate is a broad spectrum of moderately toxic organophosphorus pesticide used as insecticide on a large variety of crops including fruits, vegetables, cotton, corn and wheat. With especially the recent elimination of five highly toxic organophosphorus pesticides, chlorpyrifos has been widely used in China. Consequently, large quantities of wastewater containing chlorpyrifos have been generated from pesticide industry and lot more chlorpyrifos scattered in the depths of soils and waters in the fields. Moreover, various reports have noted that chlorpyrifos have had visible toxicity in mammalians. Therefore the high degree of persistence of chlorpyrifos in the environment and the toxic effects on humans had necessitated removal. Biodegradation has received increasing attention as an efficient and cheap biotechnological approach to cleaning up polluted environments. Several chemicals have been successfully removed from soil and aquatic environments using degrading microbes. Similarly, biodegradation has been the major mechanism for removing chlorpyrifos residues, especially for treatments of discharged wastewater from the processes of chlorpyrifos production. Previous successes in isolating Bacillus cereus strain from chlorpyrifos degradation have augmented scarce literatures on this strain of chlorpyrifos biodegradation. In order to enhance degradation efficiency, B. cereus HY-1 strain was immobilized with sodium alginate using the syringe titration method. Also biodegradation characteristics of chlorpyrifos by immobilized B. cereus strain were further investigated. While the optimal reaction time was obtained, the effects of the various parameters (e.g., amounts of immobilized biomass, pH and chlorpyrifos initial concentration) of biodegradation were studied. The results showed that chlorpyrifos were readily degraded by sodium alginate immobilized B.cereus. The appropriate concentration of sodium alginate was 2.5% (w/v), with an average particle diameter of 3 mm. The appropriate incubation time was 60 h, with maximum degradation rate of 100 mg·L^-1 chlorpyrifos. No significant increase was noted in chlorpyrifos degradation rate with increasing reaction time. Chlorpyrifos degradation efficiency was highest when immobilized biomass was 160 g·L^-1. Under ensured degradation efficiency condition, immobilized bacteria amount dropped, which also reduced production costs. Immobilized strains were broadly adaptable to a wide range of pH, with more conducive alkaline conditions for chlorpyrifos degradation. Compared with high initial chlorpyrifos concentrations, degradation rates at low initial chlorpyrifos concentrations (80 mg·L^-1 and 100 mg·L^-1) were higher. Chlorpyrifos degradation rate reached 90% at 100 mg·L^-1. But with increasing concentration, chlorpyrifos degradation rate declined. This showed that immobilized bacteria tolerance to chlorpyrifos concentration was within a certain limit. Immobilized bacteria were reusable in chlorpyrifos degradation processes. With repeated use (4 times) of immobilized bacteria, though immobilized bacteria disintegrated, the degradation rate at 100 mg·L^-1 of chlorpyrifos was up to 47%. Therefore immobilized strains showed high application values in removing chlorpyrifos residue with wide environmental adaptability ranges.