Abstract: A.C. impedance plot often shows rotation of semicircle it, complex plane, i.e. the centre of semicircle locates below real axis. However, the physical causes of that phenomenon are not yet completely clear. One possible cause is the surface roughness of electrode. In regard to metal eleetrods covered with surface film,the film can be considered as dielectric inserted between metal and electrolyte just as the case of a capacitor. Owing to dielectric dispersion, the capacitance of that capacitor in alternating electric field is a complex parameter C=C′-jC″. Literature data indicated that in the frequency range of a.c. impedance measurement both real and imaginary components of complex capacitance C′ and C″ are independent of frequency. It is postulated that the dielelctric behaviour of that capacitance might be the cause of impedance semicircle rotation. Based on equivalent circuit shown in Figure 4, it could be deduced that the semicircle equation on polar cooordinate is ρ=(R_t/cosδ)cos(θ+δ), and parameters of eqnivalent circuit can be calculated from impedance semicircle according to C′=1/ωR_t·1/(ctgθ-thδ) and C″=C′tanδ, Besides, the apex frequency on semicircle is f~*=fcosδ(ctanθ-tanδ). Above formulate can be reduced to normal semicircle in case δ=0.