Thermal stability of Ta films grown on Si(100) was investigated by in situ x-ray scattering and ex situ cross-sectional transmission electron microscopy. As a Ta∕Si(100) film was annealed at around 500°C, a uniform Ta–Si interlayer was formed at the interface. This interlayer acts as a diffusion barrier. The Ta film with the interlayer is thermally stable up to 700°C. Meanwhile, Ta films directly annealed to above 640°C exhibit no interlayer formation and transform to randomly nucleated tantalum-silicide phases. Maintaining a uniform interlayer is a critical factor for enhancing thermal stability of Ta∕Si(100) films.