In this paper, a generalized Ohm's Law of unsteady state in partially ionized plasma is derived. Various appropriate expressions under concrete physical conditions are analysed. When plasma approaches fully ionization, the generalized Ohm's Law reduces to the form given by Spitzer; and when partially ionized plasma reaches steady state, the generalized Ohm's Law reduces to the form given by Cowling and Любимов. Finally, this Ohm's Law is applied to certain problems on plasma a. c.resistivities, dispersion relations and diffusions, etc.