The authors report on a study of the transformation behavior in Ti-15-3 and Ti{sub 3}Al-Nb alloys especially on the nature of the premartensitic phenomena by electron diffraction and microscopy. The premartensitic behavior of commercial beta Ti alloys has not been reported in other papers. The microstructure of premartensitic phase, called tweed or mottled structure composed of fine streak lamella, may have significant effect on mechanical properties of hardness, strength and toughness, since the structure of the phase provides many sites of nuclear in aging. The authors will consider the aging behavior of {beta} or B2 Ti alloys caused by fine precipitates from premartensitic phase, and the phase relationship between precipitates and matrix.