Gasification process is a fairly complicated matter and using pelletized biomass for the gasification mostly results in fuel agglomeration. The pelletized biomass moving from the pyrolysis zone to the oxidation zone sticks together in lumps and disrupts entire process. In order to determine the regularities of thermal deformations, experimental research of pelletized biomass thermal deformations during pyrolysis were performed in a horizontal pyrolysis reactor from 300-900°C temperature capturing wood particle, wheat straw and wood pellet radial changes by a digital camera. Also the center temperature and the mass loss of samples were measured to determine cause of biomass thermal deformations. Observed results reveal that when increasing the pyrolysis temperature from 400-900°C, the wheat straw and wood pellets expand at the beginning of pyrolysis process and after it start to shrink, while wood particle is only affected by shrinkage. The swelling effect of pelletized samples starts decreasing over 600-650°C heating temperature and disappears when the temperature is higher than 850°C. Biomass shrinkage intensifies exponentially as the heating temperature increases till 700-750°C. However, the final shrinkage starts to decrease as the heating temperature increases from 700-750 to 900°C due to swelling of formed char. Determined phenomenon of pelletized biomass swelling explains cause of fuel adhesion in pyrolysis zone of gasifier. Besides estimated regularities of biomass thermal deformations upon pyrolysis could be used to improve the existing numerical models of biomass pyrolysis.