Nucleation behaviour of β to ω phase transformations in β-type Ti alloys has been investigated using electron microscopy in four cases; (1) along the {112}⟨11\\bar1⟩ twin boundary, at which compressive stress occurs, (2) by means of aging under tensile stressing, (3) on the effects on the amounts of quenched-in vacancy in specimens quenched-in from two different temperatures, and (4) due to cooling to 131 K. Results show that the formation of nucleation-sites is enhanced by both compressive-stress along the twin boundary and tensile-stress during aging. In a specimen quenched from a higher temperature, the formation of nucleation-sites increases. It is considered that quenched-in vacancies and their clusters become nucleation-sites of β to ω phase transformations. The phase transformation occurs due to cooling to 131 K. It is also found that the formation of new nucleation-sites cannot be reproduced and that they moved randomly.