Abstract: Organophosphorus pesticides have been widely used to control insect pests in agriculture. These compounds have been implicated in several nerve and muscular diseases in humans. Large-scale manufacturing and handling of organophosphate insecticides have caused the contamination of soils, air, surface water and groundwater across the globe. Chlorpyrifos degradation has become an increasingly important area of research in recent years. The current focus has been on the development of cost effective and eco-friendly technologies for treating polluted environments. Biodegradation has offered an efficient and cheap option for such decontamination process. Similarly, biodegradation has also been the main mechanism for removed chlorpyrifos residue, especially in the treatment of discharged wastewater from the varied processes of chlorpyrifos production. The study of the degradation kinetics was useful for understanding biodegradation processes as well as estimating pollutant concentrations. The degradation of high concentration pollutants was critical for developing practical applications of degrading bacteria. In this study, two pure Bacillus cereus strains (HY-1 and HY-2) were used in chlorpyrifos biodegradation in a batch of shake-flasks. The rate and extent of biodegradation were quantified at different chlorpyrifos initial concentrations (in the range of 40~200 mg·L 1) by B. cereus through adding chorpyrifos to mineralized medium and analyzing chlorpyrifos residue at a definite time. At the same time, the effects of incubation times and size of inoculum on chlorpyrifos degradation rate were determined. The degradation rate of high chlorpyrifos concentration by B. cereus was also measured. The results showed that the appropriate incubation time of inoculum of HY-1 and HY-2 strains were 10 h and 19 h respectively. The data suggested that inoculum incubation time significantly influenced chlorpyrifos degradation efficiency. Optimum inoculum amount for the two strains was 8% (v/v) of total volume. The effect of inoculum size on HY-1 degradation function was greater than that of HY-2 as the ratio increased from 4% to 8%. The simulated results suggested that the fit of the first-order kinetics model ln(C0/Ct)=kt was good for initial chlorpyrifos concentrations of 40 mg·L^-1, 80 mg·L^-1, 100 mg·L^-1 and 120 mg·L^-1. With further increases in chlorpyrifos concentration, only HY-2 followed the first-order kinetics model in terms of chlorpyrifos degradation. When chlorpyrifos concentration was within 40~120 mg·L^-1, HY-1 degradation efficiency constant was within 0.013 5~0.015 7. Similarly, HY-2 degradation efficiency constant was within 0.008 0~0.015 3 when the chlorpyrifos concentration was 40~200 mg·L^-1. HY-2 strain was possibly more suitable for higher chlorpyrifos concentration than HY-1 in terms of chlorpyrifos degradation, with HY-2 degradation rate relatively higher as well. The results suggested that further research was needed on the two microbes to fully understand their use as potential bio-agents in chlorpyrifos water contaminated remediation.