The deformation behavior of TiAl PST crystals with the lamellar boundaries parallel to the loading axis has been investigated in plane strain compression in comparison with the behavior in conventional compression. In conventional compression, the macroscopic deformation occurs in an anisotropic manner that the specimen dimension measured in the direction parallel to the lamellar boundaries increases, leaving the dimension measured in the direction parallel to the normal of the lamellar boundaries almost unchanged. When deformation in the direction parallel to the lamellar boundaries is restricted in plane strain compression, the increase in yield stress and work-hardening rate and the decrease in strain at failure are noted as compared to the case of conventional compression. These changes in mechanical properties are discussed in terms of the operation of additional deformation modes that are needed to generate the macroscopic strain in plane strain compression.