The accurate thickness measurement of Ultra-thin rolling metallic foil has an important role in industrial or some special applications. Unfortunately, commercial thickness meters do not provide high precision measurements non-destructively. A new spectral-domain interferometric method for measuring absolute thickness of Ultra-thin metallic foil is proposed here. The thickness is measured by differential white light spectral interferometer. Two differential Michelson Interferometers (MI) are used as basic measuring system to obtain the spectral interference fringes on the spectrometers. The spectral interference between both beams, which shows up a periodic modulation of the source spectrum with the period dependent on the OPD, serves as an illustration of a technique for measuring both OPDs and displacements in a range dependent on the source spectrum width. Therefore, the interference fringes only depend on the OPD due to the thickness of metallic foil and are unrelated to the position of the foils in the system, which is insensitive to the vibration. The spectral interference fringes are resolved over a wide spectral range and the absolute thickness of metallic foil can be calculated by measuring the OPD with a modified extremum method based on the least root mean square (RMS) deviation. The theoretical analysis and preliminary experiments indicate that the technique can measure the thickness of foils in the range of 1μm to 80μm, and it requires less than 50ms within the single measurement. Experimental results are presented.