A safe and highly accurate method is essential for determining the diameter of an optical fiber. Mechanical methods are not applicable for this work, because of the optical fiber surface damage that can result during a mechanical measurement process. On the other hand, optical measurement methods, based on the interference and diffraction effects, are known as very accurate, rapid and safe techniques for measuring the diameter of optical fibers.  In this work, we analyze the diffraction pattern, resulting from illuminating an optical fiber by a He-Ne laser, to measure the diameters of a standard optical fiber with and without the buffer layer. The laser measurement results show a very good agreement with the absolute sizes.