This chapter focuses on the transmission characteristics of single-mode optical fiber. Optical fiber is the most important invention of the signal transmission line during the 20th century. In the large capacity and long distance transmission system, the most common optical fiber is the single-mode fiber made of silica glass. The structure of optical fiber for telecommunication use is standardized by ITU. In the step-index round optical fiber, the field profile is analytically expressed in terms of Bessel functions. For generalized distributed index profile in the core, some numerical calculation methods are needed. The pulse width transmitted in a single mode fiber is broadened due to the wavelength dependence of group velocity. Since the material dispersion can be compensated for by the appropriate control of waveguide dispersion, some dispersion shifted fibers have been developed. Using these dispersion shifted fibers, a long distance large capacity transmission becomes possible.