The method for removal of silicon and boron from ultra-pure water by EDI (electrodeionization) was investigated. By investigating electric current, conductivity, flow of feed water in dilute and concentrate chambers, and pH value, the optimal conditions for removal of silicon and boron are obtained. With 1000 mu g/L SiO2 in feed water, the concentration of silicon in product water can be decreased to 2.66 mu g/L, which is currently the best result in our country. With 50 mu g/L boron in feed water, less than 1 mu g/L boron in product water can be obtained. All these results meet the requirements for ULSI (ultra large scale integrated circuits) (silicon: <= 3 mu g/L, boron: <= 1 mu g/L).