The environmental impacts of the manufacturing stage of the product life cycle are resource consumption and waste emissions. The development of an appropriate assessment method for evaluating the environmental performance of a manufacturing process, or an entire process plan, is one of the keys to realize a successful sustainable manufacturing strategy. This paper describes a new method for the environmental evaluation of a manufacturing process plan, where actual values for various environmental measures for the different operations in the plan are used to form an impact matrix. If desired, scores for each process, each environmental measure, and for the entire plan may be obtained by applying weights obtained from the analytical hierarchy process. The method has been encoded in software (Manufacturing Environmental Performance Evaluation), which provides insights into how manufacturing decisions affect the environment. A case study is used to illustrate how the assessment method may be applied and to demonstrate its applicability.