Oil spill disturbs an entire ecosystem for a quite long period of time, environment impacted by this event but less attention was drawn due to lack on documentations. This paper presents a study of the past accident related to the offshore oil and gas accident that contribute to environmental loss. It intend to assess the environmental loss quantitatively. Referring to nine cases of the worst offshore accident related to structure failure in the previous years (1969-2011), the cause of failure that contribute to oil spill and environmental loss in the previous cases was identified and categorized based on the environmental loss characteristics. Finally, data collected from the past accident is used to design the questionnaire survey in order to obtain the public perception and analysis is done by using average index of non-parametric test and analytic hierarchy process for prioritization the factor of environmental loss factor. The result shows that water pollution, air pollution, marine animal and fisherman denoted as A1, A2, B1 and C1, respectively, are very important are the most affected factor. Identifying the loss of environmental loss factor is a crucial contribution to the enhancement of the calculation of COF as well as risk assessment.