Perennial C4 grasses are candidates as energy crops because they have a low demand for nutrient inputs and show higher biomass yields on relatively poor quality land sites with longer persistency and have the merit of increasing the soil carbon content as well as an effect on the stability and cover value for wildlife.  One of the most intensively investigated potential new energy crops is the genus of Miscanthus that is native to Asian countries including Japan.  A breeding program for Miscanthus ssp. to develop an energy crop has been started at Hokkaido University, Japan recently.  We have collected more than 500 accessions of Miscanthus spp., mainly M. sinensis, distributed all over Japan. Evaluation of biomass potential using some of accessions of M. sinensis was carried out in the field.  Much genetic variability was found in morphological and flowering traits and biomass potential among accessions. Accessions collected from northern areas such as Hokkaido showed earlier flowering time and deeper autumn dormancy than those from southern areas. M. sinensis accessions from other areas of central Honshu and Shikoku Islands with high altitude were tolerant to Hokkaido’s cool environment and showed high biomass potential.  Much genetic variability would be useful for breeding program of Miscanthus as feedstock materials for the well adaptation and high productivity in the cool environment.