This paper discusses the significant percentage increase of environmental impact generated in the manufacturing phase as a result of changes in the key material selection during the product design process. The findings in this paper are important for product designers. They need to pay extra attention when choosing plastics or metals when assessing design alternatives that can reduce the environmental impact. Four products were analysed in this case study namely water pump, hot pot, plastic kettle and stainless steel kettle. The environmental impacts of these four products are assessed by Life Cycle Assessment (LCA) and the CML approach from Institute of Environmental Sciences is adopted as the Life Cycle Impact Assessment method.