Abstract: A novel method to measure the cladding thickness of side-polished fiber (SPF) was demonstrated by using the digital holographic imaging technology. Using angular spectrum method and accurate least-squares phase unwrapping method，SPFs distribution of phase was reconstructed. Based on the reconstructed distribution of phase, the cladding thickness of SPF could be directly measured. Compared to the results measured by scanning electron microscope (SEM), the relative error is less than 0.5%. This is a non-destructive, real-time and direct measurement, it can reduce the errors due to fibers asymmetries and edge diffraction. This method could also be used to measure other special optical fibers such as photonic crystal fiber and nano-fiber.