We report the experimental results of geometric parameter measurement of the single-mode fiber by use of digital holographic microtomography technology. Tomography of the single-mode fiber is performed by a filtered back-projection algorithm and a Fourier diffraction algorithm, respectively, to reconstruct the three-dimensional (3-D) map of refractive index. According to this map, we can further get the dimensions of the single-mode fiber by the relate edge detection algorithm of image processing. Experimental results show that compared to the filtered back-projection reconstruction, diffraction tomography based on the Rytov approximation resembles the dimensions of the single-mode fiber. It provides a new way for the nondestructive and on-site measurement of tiny weakly diffracting objects with a cylindrically symmetric structure, such as the single-mode fiber.