In general, petrochemical industries are composed of very complex systems with internally recurring unit processes where the upstream input of a unit process may include the downstream output of the same unit process. Therefore, it rather difficult to identify and quantify the inputs/outputs for those internally recurring unit processes, respectively. In this study, an objective and consistent calculation method which is applicable to complex systems with internally recurring unit processes are proposed to determine the inputs/outputs associated with each unit process by using simultaneous equations. In this case, unit processes linked another one are considered as variables and materials flows are considered as constants. The LCI results from the above methodology will be analyzed and compared using hypothetical examples.