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A novel method for measuring modal power distribution (MPD) in multimode fibers is proposed using built-in tilted fiber Bragg gratings (TFBGs). For each degenerated guided mode group, a portion of the guided power can be coupled to the corresponding radiation mode through a TFBG imprinted into the same fiber, and then measured separately with a spatial filter. The power coupling coefficients between the guided and radiation modes are obtained by solving a set of linearly independent equations acquired under a number of launching conditions. With these coefficients obtained, the MPD under any excitation condition can then be obtained by simply measuring the powers of radiation modes. The proposed method is successfully demonstrated with a few-mode fiber.