This paper describes a Life Cycle Assessment (LCA) done to evaluate the relative environmental performance of magnesium (Mg) and aluminum (Al) automatic transmission cases. Magnesium is considered a lighter weight substitute for aluminum in this application. Light weighting of vehicles increases fuel economy and is an important vehicle design metric.The objective of this LCA is to quantify energy and other environmental trade-offs associated with each alternative for material production, manufacturing, use, and end-of-life management stages. Key features of the inventory modeling and the data collection and analysis methods are included in this paper along with life cycle inventory profiles of aluminum and magnesium alternatives. The life cycle inventory (LCI) was interpreted using a set of environmental metrics and areas needing further research were identified. A qualitative cost assessment was done in conjunction with this LCA to highlight potential cost drivers.