Due to the high potential of petrochemical industry in the creation of environmental impact, environment impact assessment of the development of petrochemical industry is very important. The aim of this study is to provide and test a fuzzy model for environmental impact assessment of petrochemical industries as a decision support system in planning process and the development of petrochemical industry. To test this model, the Lorestan province was chosen as the study area. In order to assess the impact of petrochemical industry in Lorestan, the combination of RIAM method and fuzzy theory was used. First, using check lists and holding meetings with experts, the type of petrochemical industry impacts on environmental factors was determined. Next, the effects of project activities on environmental factors were scored; the fuzzy scoring was done using defined criteria in MATLAB software. Environmental components considered in the method used were divided into 4 main factors including physicalchemical, biological-ecological, social-cultural and economic-functional and 33 sub-factors, assessment of nonimplementation, construction and operation stage was carried out individually. The results show the most important environmental impacts of petrochemical projects, water and soil pollution, air and noise as well as its socioeconomic consequences. The results showed that the operation in petrochemical industry will create more important implications than the construction stage, some of them in terms of lack of control and environmental management, are irreversible and damaging.