Micromechanics theory is used to analyze the yield stress of polysynthetically twinning (PST) crystals of ?TiAl as a function of the loading angle between the lamellar and the loading axis. The numerical results of yield strength, based on considering the lamellar microstructure and the deformation mechanisms such as ordinary slip systems and true twinnings in PST crystals composed of and phase, is obtained by using calculating of resolved shear stresses in all slip systems based on micromechanical method. The emphasis was put on discussing the influence of prism slip systems in ? phase on the yield stress of polysynthetically twinning (PST) crystals of ?TiAl. The results indicates that the slip systems in ? phase have, to a certain extent, influences on the yield strength of PST crystals, especially at small angles ?directions.