The influence of conditioning regimes of some chosen plant materials and their mixtures upon the density of obtained pellets was presented in the paper. Hydrothermal conditioning procedures were compared to that applying addition of water only. It was confirmed that the pellets density was significantly influenced by the steam treatment conditions (p<0.05), and it decreased with a rise in the temperature of steam used. At the same time, the rising steam pressure (for temperatures within 50-70 °C) caused the linear increase in the analysed feature. Contrary, when water was added instead of steam treatment, for all materials used in the study, strong negative linear relations were noticed (r>-0.93), and the average density of pellets ranged from 438.3 do 756 kgźm-3.