An approximate relationship of current-potential for a steady-state quasi-reversible reaction at an oblate hemispheroidal microelectrode has been deduced by using rotational elliptic coordinates. A method for measuring the heterogeneous rate constant and formal standard potential from a stationary voltammogram at an oblate hemispheroidal microelectrode has been proposed. As an application, a near-oblate-hemispheroidal microelectrode has been made by electrodeposition of mercury on a platinum disc microelectrode. The heterogeneous rate constant and formal standard potential of 2,3,5,6-tetrachlorobenzoquinone in acetonitrile have been determined by the method described in this paper. The rate constant is near to, but smaller than, that measured by cyclic voltammetry reported previously. The formal standard potentials at different oblate hemispheroidal ultramicroelectrodes coincide with each other.