We report martensitic shear transformation of strongly segregated block copolymer micelles on face-centered cubic (FCC) lattices to hexagonally close-packed (HCP) structures elucidated by X-ray scattering characterizations. The initial FCC crystal structures of the block copolymer micelles were prepared by direct dissolution of poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) diblock copolymer in the water at 25 °C, and the FCC crystal domains were shear-aligned during the sample preparation process for the X-ray scattering measurements. Heating the shear-aligned FCC crystals of the PB-PEO micelles above 80 °C initiated the transformation to HCP structures, which are also found stable at 25 °C when cooled from the transition temperature. Remarkably, we found that the HCP crystal domains are also aligned, and this suggests that the FCC-to-HCP phase transition has occurred by the martensitic shear transformation. Scattering pattern analysis reveals that the martensitic shear transformation proceeds by preferentially dislocating a specific set of two-dimensional hexagonal close-packed (2D-HCP) layers among four equivalent 2D-HCP layers of the initially shear-aligned FCC crystals. We believe that the selective martensitic shear transformation originates from the orthorhombic-like morphology of the FCC crystal domains formed by slip dislocations and stratifications of initial FCC crystal grains during the sample preparation process. In the shear-aligned FCC crystals with the orthorhombic-like crystal domains, the specific 2D-HCP layers chosen for the martensitic shear transformation have the least area of dislocations, i.e., the least kinetic energy barrier, for the FCC-to-HCP phase transition and appear to be preferentially utilized. These findings show that the size and morphology of crystal domains are critical to the formation, stability, and transformation of crystalline structures and consequently control the polymorphism of solid compounds.