Single-phase hexagonal close-packed structure of the ScYLaGdTbDyHoErLu high-entropy alloy was studied in detail. The applicability of the rule of mixture was analyzed with respect to the lattice constant, mechanical parameters, elastic properties, melting point, and hardness of the alloy. Significant tension-compression asymmetry has been found and explained by the strength differential effect during the uniaxial tests. Numerous deformation twins and high densities of stacking faults can be observed from the morphological characterization by a transmission electron microscope, which governs the main deformation mechanism during the plastic deformation in the current high-entropy alloy.