The theoretical modeling and characteristics of biosorption using radionuclide-absorbing microalage (Chlorella vulgaris) in low-level radionuclide-contaminated water are implemeted. Radioactive materials such as cesium (Cs) and strontium (Sr) passing through holes in semipermeable membrane are absorbed by microalgae. Basic experiments are performed in order to investigate equilibrium biosorption conditions between C. vulgaris and radionuclides. Based on the experimental results, biosorption characteristics of C. vulgaris on each radionuclide are theoretically established by Langmuir isotherms. A kinetic model on the biosorption process of the proposed membrane system has been developed by mass balances. And simulations of the biosorption efficiency over time have been performed to predict the removal characteristics of radionuclides on the biosortion process.