This article aims to investigate the effects of a small amount of ionic liquid on the effective electrostatic potential between two charged plates immersed in an organic solvent. Utilizing classical density functional theory, we studied the variation patterns of the effective electrostatic potential with respect to system parameters such as solvent polarity, bulk concentrations of solvent and ionic liquid, and counter-ion electric valence. We discovered new features that differ from the effective electrostatic potential in aqueous electrolyte solutions. These include: +1:−1 type ionic liquid induces like-charge attraction; as the solvent polarity increases, the effective electrostatic potential changes from attraction to repulsion at small surface separations; within the like-charge attraction potential well, univalent counter-ions can induce a repulsive barrier, while high valence counter-ions can induce a second potential well at low ionic liquid bulk concentrations. By decomposing the effective electrostatic potential into entropy-driven and energy-driven components, we have gained a mechanistic understanding of the rules governing the variation of the effective electrostatic potentials. The present findings offer guidance for electrostatic self-assembly, colloid stabilization, protein folding, and molecular recognition, among others.