Aging of metastable β-Ti alloys leads to the fine precipitation of co phase, resulting in a marked increase in strength accompanied by a significant loss in ductility (omega brittleness). Due to their small size and high number density, the observation method for co phase has been limited to transmission electron microscopy (TEM). The present report describes the result of 3D observations of the isothermal co phase by a newly developed FIB/SEM system In this system, a SEM column and FIB column are arranged orthogonally; this arrangement ensures that the surface sectioned by FIB can be perpendicular to the incident electron beam It was possible to observe the co particles of less than 10 nm with a very high contrast by an in-lens secondary-electron (SE) detector. The ω particles appeared darker than the matrix in the SE images. This may be due to the lower Mo concentration in the co particles than in the surrounding matrix. It was revealed that the shape of the particle varies with the size. The larger particles (about 100 nm) are ellipsoidal while the smaller ones are spherical. In addition, by serial sectioning (cut &. see),it was possible to obtain a 3D image for the volume of a micrometer cube containing numerous co particles, which is much larger than the volume observable by a TEM As a summary, this new FIB/SEM system is an extremely powerful tool and provides an opportunity to conduct an accurate quantitative assessment on co phase formation, which has never been done.