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In this study, we report the effect of elemental combinations on the friction stress and Hall-Petch relationship in medium entropy alloys (MEAs) and high entropy alloys (HEAs) which are defined as the alloys composed of four or less and five or more principal elements, respectively, with (near-) equi-atomic concentrations. The MEAs (CoCrFeNi, CoCrNi, etc.), which are subsystems of equi-atomic CoCrFeMnNi HEA, were highly deformed by high-pressure torsion (HPT) and subsequently annealed at different temperatures. The specimens with fully-recrystallized microstructures of FCC single-phase with various mean grain sizes down to sub-micrometer scale were obtained. Subsequently, tensile tests were performed at room temperature to obtain precise Hall-Petch relationships and friction stresses of the materials. Co₂₀(CrNi)₈₀ was successfully predicted as the alloy showing the highest strength among the MEAs …