Abstract: In this paper the formation of niobium colloid in 0.5mol/l HNO3 of various concentrations of niobium by electrodialysis is studied. Using five cells separated by four semipermeable membranes, the electrode action on the medium in the mid-cell is weakened, and the concentration of nitric acid in the mid-cell is almost constant during the test process. It is observed that the form of niobium colloid formed varies with the "apparent dialysis ratio" . The electric character of colloid particle is given.The behaviour of electrodialysis of niobium with the concentration of 1×10-6 mol/l is the same as trace niobium. In this case, no niobium colloid is formed in the solution. If niobium concentration is in the range of 7 to 9×10-6mol/l, the "apparent dialysis ratio decreases significantly, which shows that niobium colloid is being formed partly. If the concentration of niobium is higher than 1×10-6mol/l in the solution, "apparent dialysis ratio" decreases sharply and appears in the curve with two "plateaus" .The "plateaus" show that the colloid particles of niobium are in different sizes.