Solid solution strengthening effect was investigated in Al_xCoCrNi medium-entropy alloy (MEA) with different Al content (x = 0–7 at.%). The result of X-ray diffraction indicates that the fully recrystallized phase was a single face-centered cubic (FCC) structure for all compositions. The average grain size regarding a twin boundary as a grain boundary was approximately 16 μm. Tensile testing shows that the yield strength increases with the addition of Al. Here, Labusch's approach to random solid solution alloys is applied to confirm the strengthening mechanism in Al_xCoCrNi MEA by using lattice constants and elastic moduli. The experimental results were found to be in good agreement with quantitative predictions of the solid solution contribution in Al_xCoCrNi MEA.