In the context of increasing needs of lignocellulosic biomass for emerging biorefinery, miscanthus is expected to represent a resource for energy production. Regarding biogas production, its potential may be improved either by genotype selection or pretreatment. Eight different miscanthus genotypes belonging to Miscanthus × giganteus (FLO, GID and H8), M. sacchariflorus (GOL, MAL, AUG, H6) and M. sinensis (H5) species were first compared for biomass composition and potential methane. In a second time, alkali pretreatments (NaOH 10 g 100 gTS−1, CaO 10 g 100 gTS−1) were applied at ambient temperature and high solid content, in different conditions of duration and particle size on the genotype FLO presenting the lowest methane potential. The methane potential varied between miscanthus genotypes with values ranging from 166 ± 10 to 202 ± 7 NmLCH4 gVS−1. All of the studied pretreatments increased the methane production up to 55% and reduced Klason lignin and holocellulose contents up to 37%. From this study, NaOH was more efficient than CaO with an increase of the methane production between 24 and 55% and between 19 and 30%, respectively.