A novel transmitted-light differential interference contrast(DIC)system is used for nondestructive measurement of the refractive-index profile(RIP) of an optical fiber. By means of this system the phase of a measured light beam can be modulated with an analyzer. and the phase distribution of a fiber is obtained by calculation of the various interference patterns. The measurement theory and structure and some typical applications of this system are demonstrated. The results of measuring RIPs in graded-index are presented. Both the experimental results and theoretical analysis show that the system takes the advantage and high index resolution and of sufficient measurement analysis show that the system takes the advantage of high index of the optical fiber. The system has strong ability to overcome environmental disturbance because of its common-path design. Moreover, one can use the system to measure the RIP along the fiber axis and acquire an image of the three-dimensional RIP of the fiber.