Although grass Miscanthus × giganteus biomass is currently used mostly as a raw material for direct combustion, an increasing consideration is now given to its potential use in production of biogas, namely biomethane. The aim of this paper is to determine the yield of dry-matter (DM) and organic dry matter (oDM) from the crop Miscanthus × giganteus in three different harvest times and at three different locations, and, based on these values, to calculate the biomethane yield on the abandoned agricultural surfaces in Croatia. The investigations which were conducted for this purpose determined the average values of dry matter yield (24.77 t/ha) and organic dry matter yield (24.11 oDM/ha) as well as the biomethane yield of 5976 Nm3/ha. In relation to total natural gas consumption and percentage of the abandoned agricultural land, Croatia could replace between 1.6% to 4.8% of the consumed natural gas by introducing Miscanthus × giganteus grass on 5% to 15% of the abandoned agricultural lands in three of its counties (Sisačko-moslavačka, Karlovačka and Ličko-senjska).