In this study, an offshore process FEED(Front End Engineering Design) method is systematically established to perform integrated process engineering for topsides systems of LNG FPSO(Floating, Production, Storage, and Off-loading unit) based on the concepts and procedures for the process FEED of general offshore production plants. First, various activities of the general process FEED engineering are summarized, and then the offshore process FEED method, which is suitable for application to all types of offshore oil and gas production plants, is proposed. Second, an integrated process engineering environment is built based on the proposed FEED method. Finally, the integrated process engineering environment is applied to topsides systems of an LNG FPSO in order to verify the validity and applicability of the proposed FEED method. As a result, it is shown that the proposed FEED method can be applied to the process FEED engineering of FPSOs and moreover will be able to contribute to perform successful offshore projects in the future.