A study was conducted to construct functions for aboveground biomass of different components (leaves, stems, and branches) of hazel (Corylus avellana L.), rowan (Sorbus aucuparia L.), and alder buckthorn (Frangula alnus L.) growing naturally under the stand canopy. The functions were designed to calculate the potential amount of biofuel produced from underbrush species in Lithuanian forests. Biomass production of common underbrush species was estimated in seven stands located in in southwestern Lithuania. The ages of the stands varied from 55-70 to 80-100 years. The biomass relationships with morphometric parameters were determined, including stem diameter at 1.3 m and 0.3 m above ground level, stem height, and crown length. The study findings showed that biomass relationships for underbrush species were not comparable to the same relationships for canopy trees. As the biomass changes for canopy trees are mostly caused by competition, and the growth of underbrush species depends more on the light regime. Therefore, stem diameter (D) rather than stem height (H) should be better used for estimating total aboveground biomass of underbrush.