Orbifold branes in string theory are investigated, and the general field equations both outside and on the branes are given explicitly for type II and heterotic string. The radion stability is studied using the Goldberger Wise mechanism, and shown explicitly that it is stable. It is also found that the effective cosmological constant on each of the two branes can be easily lowered to its current observational value, using large extra dimensions. This is also true for type I string. Therefore, brane world of string theory provides a viable and built-in mechanism for solving the long-standing cosmological constant problem. Applying the formulas to cosmology, we obtain the generalized Friedmann equations on the branes.