Concrete blocks prepared with Portland cement and miscanthus-based aggregates were prepared in order to check if the miscanthus genotype may influence their mechanical properties and to perform an environmental assessment. To produce lightweight, load-bearing concrete blocks using miscanthus stem fragments as aggregates in a single mixing method turned out to be impossible, although trying to optimize the concrete formulation. The results show that genotypes and size of miscanthus fragments controlled the mechanical properties of the final blocks. The lower was the amount of light elements such as leaves and sheath, the better were the mechanical properties of the blocks. When comparing genotypes with the same leaf/stem ratio, it was not possible to see a correlation between the biochemical composition of the stem and the compressive strength of the blocks. A probable explanation is the small variation of biochemical composition between genotypes. Using life cycle analysis tools, miscanthus block were not found to be competitive with conventional alternatives (concrete block and lightweight pumice block) when trying to increase compressive strength above 3 MPa. However, compared to non-load bearing alternatives (light clay brick), blocks integrating miscanthus had a better global environmental performance mainly due to a favorable climate change impact. The present work also points out the risk of decreasing the environmental performances when cultivating the crop on land in competition with food, because of the impacts of indirect consequences of Land Use Change.