The strength and electrical conductivity balances in Cu/martensite (α′) steel multilayered sheets with various volume ratios were evaluated. Furthermore, the measured tensile properties and electrical conductivity were compared with the values estimated from the rule of mixtures, and the reason for the difference between the measured and estimated values was discussed based on the deformation and fracture behaviors. The multilayered sheets exhibited excellent strength-electrical conductivity balances superior to those of conventional Cu alloys, and their strength and electrical conductivity can be controlled over a wide range by changing the volume fraction of α′ steel. The electrical conductivities of the multilayered sheets with different volume ratio were approximately identical to the estimated values based on the rule of mixtures. However, the ultimate tensile stresses of the multilayered sheets with lower volume fractions of the α′ layer were slightly lower than the estimated values. A significant strain concentration occurred within the α′ steel layer in the multilayered sheet with the lowest volume fraction of α′ steel. Furthermore, cracks and/or voids were formed in the α′ steel layers even during the uniform-deformation stage. Therefore, the early fracture of the α′ steel layers caused lower ultimate tensile stresses in the multilayered sheet with low fractions of the α′ steel than the estimated value based on the rule of mixtures.