In order to cope with severe competition in the global market, the local area network installed inside the plant site has been connected with the Internet for exchanging business information among globally distributed plant sites. Though the information systems have given benefits through such open architecture, there are various risks as the result of the application of network technologies. Much attention, therefore, has to be paid for chemical plant failures caused by triggering from outside parties. One of the common technological interests is a risk analysis method of network events that allows illegal access to the digital control system (DCS), In this paper, the authors present a unified approach to generate fault tree and event tree structures between a network access and an anomaly of process plant. To illustrate the validity of the proposed method, a typical networked chemical plant is taken as an example and analyzed. The present research will make clear what parts are most important for the plant security.