Properties of bipolar electron and ion flow in a relativistic planar diode are investigated within the framework of the cold‐fluid‐Poisson equations. Defining the parameter q by q=(Ji/Je) [(mi/2Zme)]1/2, the electron current density Je is expressed in terms of the diode voltage, the anode‐cathode gap, and the parameter q satisfying q≤qs=(γ0+1)1/2/2. Here me is the rest mass of electrons, Z and mi are the charge state and the rest mass, respectively, of ions, Ji is the ion current density, γ0 is the relativistic mass ratio of electrons at the anode, and the extreme value q=qs represents the space‐charge limited ion flow. From the analysis, it is found that presence of the ions in the diode significantly enhances the electron current, although the ion current is less than few percent of the electron current. In an appropriate physical parameter regime, the electron current with counterstreaming ions is more than twice of that without ions.