The general dispersion formula of the single-mode fiber with symmetric index profile is derived in this paper. Total dispersion of step-index single-mode optical fiber with various parameters is calculated accurately, and compared with related measurement reported in the foreign paper. Especially, dispersion behaviour of step-mode optical fibers at two low-loss windows, 1.3μm and 1.55μm, is studied. All calculation is accomplished with type DJS-8 computer.