This paper presents an analytical approach to establish a relationship between the voltage variation and distributed generation (DG) integration for the planning and operation of distribution networks with DG. The proposed approach is mainly based on the derivation of a voltage variation formula for distribution networks with DG and the consideration of the worst case scenario, which establishes a relationship between the amount of voltage variation and maximum permissible DG. Some recommendations are presented based on the worst case voltage variation formula and DG integration to counteract the voltage variation effect. The relationship between the connection cost and voltage level is also presented in this paper. The feasibility of the proposed approach is validated by comparing the voltage profile obtained from the derived formula to that with the existing power system simulation software.