Investment in Chemical Process Industries for improving their safety requires considering risk level, environmental effect, cost and many other aspects simultaneously. This paper focuses on a new systematic method of finding the most cost-risk-effective investment scenario set. The method uses the automatic accident scenario generation technique first to find a set of the most dangerous scenarios. Then it uses the multiobjective optimization method to decide the priority of the investment. These computations includes considering many constraints such as limited budget, environmental requirements and social demands. The Styrene Monomer plant case study proves the practical use of this integration method of accident scenario generation and multiobjective optimization. (C) 2006 Elsevier Ltd. All rights reserved.