In the traditional designs against contamination of transmission line insulators, the maximum insulator contamination severity and insulator surface wetting are assumed as the most severe conditions. Then, the required number of insulator units per string is determined, where a safety margin of insulator unit(s) is usually allowed based on knowledge obtained at sites and in laboratory tests. Other insulator designs may be created by considering the probabilistic distributions of factors affecting insulator flashover, such as contamination severity, surface wetting, and flashover voltage. Based on this concept, flashover risk, which indicates flashover probability in a certain period, is used in this study as an evaluation function of reliability. The characteristics of flashover risk are discussed, and the proposed probabilistic method is applied to the insulation design of a 500kV ac transmission line. Assuming a certain acceptable flashover risk, the number of cap-and-pin insulator units per string is calculated and compared with that obtained using the conventional deterministic method. When the number of insulator units per string is fixed, flashover risk can determine the number of flashover events in a certain period, such as a year. Findings suggest that the probabilistic approach is a potential insulation design method under contamination conditions.