Abstract In this study, the number of stomata per unit area was determined in the perennial energy grass of two genotypes Miscanthus × giganteus (Greef et Deuter) and M. sinensis ‘Tatai’ on arable land in the South-western Slovakia. The dynamics of stomatal density per mm2 was determined by non-destructive method in three randomly selected clumps. The number of stomata per mm2 of area varied from 125 ±23 to 327 ±47, stomatal length ranged from 33.9 ±4.7 μm to 35.3 ±3.4 μm, and stomatal width ranged from 17.5 ±2.3 μm to 18.8 ±2.8 μm in the two genotypes. We confirmed ontogenetic heterogeneity of leaf area size and stomatal size parameters.