Na-alginate was selected as an entrapping agent to immobilize ammonia-oxidizing bacteria in this experiment, designed to explore optimal entrapping condition and to compare immobilized bacteria with non-immobilized bacteria in ammonium removal efficiency. Results show that when Na-alginate was at 4%, and CaCl₂ at 3%, an optimal entrapping condition was achieved with bacteria entrapment reaching at 3.0 g L^-1, and ammonia removing rate at 80%. Compared with non-immobilized cells, the immobilized ones were more adaptable to variation of temperature and pH, as well as exposure to organic matter and toxicants.