Industrial symbiosis (IS) activities within an industrial complex can enhance environmental and economic efficiencies. This study proposed a simplified life cycle assessment (LCA) methodology as an effective tool with which to evaluate the reduction of environmental impact of IS activities. In this method, the variation of resources and energy consumptions before and after IS activities are proposed to simply evaluate its lifecycle environmental impacts. The simplified LCA was applied to an IS case between an industrial waste incineration plant and a steam production plant of a paper mill firm in the metropolitan city Ulsan. The system boundary and inventory were set for this IS, and an environmental impact assessment was carried out by standard and proposed LCA methods. The results showed that the environmental impacts after IS decreased in all impact categories with regard to the consumption reduction of boiler fuel used at the paper mill. Furthermore, the performance of environmental improvement activities such as IS can be simply evaluated by only considering the input and output changes in the environmental improvement activities. In this IS case study, the environmental impact was decreased by 11.7% (weighted impact base). This was due to the utilization of waste heat generated from an incineration plant in the process of the paper mill firm.