Geometric parameters of multifold fiber are measured, and an exact and applied fiber diameter measurement method is proposed based on the digital image processing. The method acquires the fiber picture via optical microscopy system and image acquisition system, and then processes the data via data fitting by computer. The diameter of vacant core fiber and non-communications fiber is measured, and then the data error is analyzed. The experimental results show that the proposed method is simple and easy, and it can be applied to some special areas where direct measurement is unfeasible.