This paper investigates grounding impedance reduction with various shapes of grounding electrodes in comparison with a conventional vertical rod based on field measurements and FDTD simulations. It is found that a vertical rectangular conducting plate is effective to reduce the transient and steady-state impedance less than a half of that of the vertical rod. A circular conducting plate parallel to the earth surface is also effective to reduce the steady-state impedance, but the transient impedance is nearly the same as that of the vertical rod. The rate of the reduction is somehow proportional to the surface area of the electrode, but tends to saturate as the area increases. The transient impedance is rather independent of the earth resistivity and permittivity, but tends to inversely proportional to the wavefront duration of an applied current.