XPS (X-ray Photoelectron Spectroscopy) or ESCA (Electron Spectroscopy for Chemical Analysis) can accurately measure the thickness of ultra-thin films thinner than 2 rim and its precision is +/- 0.1nm. XPS remedy the defection of TEM that is difficult to determine the thickness for films thinner than 1 rim, but XPS is not accurate for films thicker than 10 rim. This paper aims at reviewing the application of XPS in determining thickness of ultra-thin films.