The levels of emerging chemicals have increased dramatically during the last two decades, posing problems for human and environmental health. The lack of a rigorous control of pharmaceutical discharges into natural water bodies is a concern for limnologists and ecotoxicologists because of the possible harmful effects of these chemicals on non-target organisms. The present study was performed to assess the chronic toxicity of a pollutant to an aquatic organism under different food densities and to screen out sensitive endpoints for monitoring ibuprofen pollution with rotifers as test animals. In particular, we studied the effects of varying ibuprofen concentrations (0, 1, 10, 100, 1000, and 5000 μg/L) on the life table demographic parameters, including life expectancy at hatching, average lifespan, generation time, net reproductive rate, intrinsic rate of population increase, and proportion of the sexual offspring of Brachionus calyciflorus fed on different densities (1.0×10⁶, 2.0×10⁶, and 4.0×10⁶ cells/mL) of Scenedesmus obliquus. The results showed that compared to the controls, 100-5000 μg/L of ibuprofen significantly decreased the life expectancy at hatching and average lifespan, 100 μg/L of ibuprofen decreased the generation time, but 1.0 μg/L of ibuprofen increased the net reproductive rate, and 10-5000 μg/L of ibuprofen increased the proportion of sexual offspring of B. calyciflorus under the algal density of 1.0×10⁶ cells/mL. When S. obliquus density was 2.0×10⁶ cells/mL, 10-5000 μg/L of ibuprofen decreased the life expectancy at hatching and average lifespan, 1000 and 5000 μg/L of ibuprofen decreased the generation time, but 1, 10, 1000, and 5000 μg/L of ibuprofen increased the intrinsic rate of population increase, and 1 and 1000 μg/L of ibuprofen increased the proportion of sexual offspring. When S. obliquus density was 4.0×10⁶ cells/mL, 100-5000 μg/L of ibuprofen decreased the life expectancy at hatching, average lifespan, and generation time. The S. obliquus density had significant effects on the generation time, net reproductive rate, and intrinsic rate of population increase (P < 0.05). The ibuprofen concentration had significant effects on the life expectancy at hatching, average lifespan, generation time, net reproductive rate, and intrinsic rate of population increase (P < 0.05). The interaction between S. obliquus density and ibuprofen concentration had significant effects on the life expectancy at hatching, average lifespan, and proportion of sexual offspring (P < 0.05). When the S. obliquus density of 2.0×10⁶ cells/mL was used in association with the range of tested ibuprofen concentrations, significant dose-effect relationships were observed between the ibuprofen concentrations and life expectancy at hatching, average lifespan, and generation time, and these relationships could be described as y=2.419×10^-6x²-0.015x + 79.818, y=2.419×10^-6x²-0.015x + 67.818, and y=1.229×10^-6x²-0.007x + 57.941, respectively. When the S. obliquus density was 4.0×10⁶ cells/mL, significant dose-effect relationships were observed between the ibuprofen concentrations and life expectancy at hatching, average lifespan, and net reproductive rate, and these relationships could be described as y=2.483×10^-7x²-0.005x + 77.624, y=2.483×10^-7x²-0.005x + 65.624, and y=- 5.434×10^-7x² + 0.002x + 14.530, respectively.