AbstractDeformation mechanism of a new Ti-16.6Nb-6Co-5.1Cu-6.5Al (at%) alloy is studied using scanning and transmission electron microscopy. The alloy consists of micrometer-sized β-Ti dendrites and an ultrafine-eutectic composed of β-Ti and TiCo phases. The yield strength of the alloy (1.1 GPa) is comparable to that of the metallic glass composites and is coupled with large tensile ductility of about 11%. Transmission electron microscopy analysis reveals that slip lines formed during deformation in the dendrites penetrates the eutectic resulting in formation of a stepped interface and an extra area serving to accommodate shear strains. The β-Ti eutectic component can deform plastically to a high degree supporting deformation of TiCo. The results suggest that microstructural design of the eutectic is important for controlling tensile ductility of dendritic-ultrafine eutectic alloys.