Abstract: The coulombs of cathodic reduction can be measured when the high pure iron electrode passivated first in 0.5 mol L~(-1) H_2SO_4 for ten minutes was swept towards the cathodic direction. It is mainly due to the reduction of oxide film on the surface. A very small amount of Cl~- can obviously decrease the reduction current. The primary effect of Cl~- is its arreting the formation of passive film and reducing the stability of the passive film.