Making correct design decisions during the early stages of the engineering design process is increasingly seen to be important, as changes during the later stage can be costly. Life Cycle Assessment (LCA) is used as a method to evaluate the design from 'cradle to grave'. In concept design, decisions are made that have a most significant influence on the life cycle, but at this stage the lack of detail makes LCA very difficult if not impossible. This thesis introduces a method that enables an 'order-of-magnitude' life cycle assessment during the concept stage of the design process. This is achieved by modelling the life cycle inventory as a function of design parameters for complete product families used in engineering design. The hypothesis is that relatively few so-called life cycle parameters determine the largest part of the life cycle inventory. Furthermore, design parameters are related to life cycle parameters, which are mathematically modelled. Design parameters are chosen so that they can be estimated early during the design process. The models of the life cycle parameters are expressed in terms of upper and lower limits, summarising data from many product families. More detailed models describe the relationships of single product families. The method is suitable for software implementation, which will especially aid the handling of sensitivity analysis. Two case studies (sealed lead acid batteries, three-phase asynchronous motors) are used to illustrate how the life cycle parameters are related to the design parameters. An overall outline of how the method is implemented into the overall design process completes the thesis (evaluation of parallel and series configuration hybrid electric vehicle).