Life cycle assessment (LCA) is the most scientific tool to measure environmental sustainability. Polygeneration is a better option than single-utility generation due to its higher resource utilization efficiency and more flexibility. Also biomass based polygeneration with CO2 capture and storage may be useful being 'net negative' greenhouse gas emission option. But this 'negativity' should be studied and confirmed through LCA. In this paper, cradle-to-gate life cycle assessment of a straw based polygeneration without and with CO2 storage is studied. Results show that captured CO2 of this polygeneration should be stored to get a net negative energy system. However, biomass distribution density, ethanol production rate and CO2 transportation distance affect the net GHG emission. For this polygeneration system, exergy based allocation should be preferred. (C) 2016 Elsevier Ltd. All rights reserved.