Large size polysynthetically twinned crystals of Ti-46Al-8Nb alloy with a parallel lamellar microstructure were successfully prepared using a Ti-43Al-3Si seed by our new operation.A large amount of colum-nar B2 phase paralleling to the growth direction was found in the final lamellar microstructure.Higher growth rate (＞30mm/h) led to the failure of seeding process.Based on these results,a new mechanism is proposed to describe the seeding process of the hypo-peritectic TiAl alloys.The peritectic α phase is suggested to directly nucleate from the melt,and then act as nucleus for transformed α phase in the sub-sequent β to α transformation.At the higher growth rate,the appearance of β phase secondary dendrites and homogeneous nucleation lead to the failure of seeding process.High Nb addition leads to a large amount of residual β phase,and these β dendrites finally evolve into B2 phase.The room temperature tensile elongation was measured to be 11.9-18.5％ for Ti-46Al-SNb PST crystals,which is the highest ever reported value for TiAl based alloys.