The granulometry of polystyrene particles of two different raw material brands (material B - Styropor from BASF and material C - NF from StyroChem) and the impact of the raw material particle distribution on the uniformity of the intermediate product were studied for large size blocks. Spatial uniformity of polystyrene blocks is important characteristic of product quality. Excess non-homogeneity of mechanical properties of produced blocks leads to mechanical deformation and defective end product. The uniformity of produced large size polymer blocks were characterized by the compression stress at relative compression strength of 10%, density, and microphotography of samples taken over produced block volume at defined places of spatial lattice. Block dimensions are 1000x1200x4000mm; 315 probes of 50x50x50mm from each block were cut according with regular lattice 250x200x500mm. Average diameter of raw material B particles is 0.7-1.0mm, but for raw material C – 0.5-0.7mm. Material C contains higher percentage of small particles than material B. In the preheating stage both material particles are expanding proportionally to particle diameter. Mass of blocks are 157.6 and 152.5kg respectively for material B and C. Density of block B probes are higher with higher values of density at the central part of block (32-35kg/m3). Densities of block C probes are lower and distributed very asymmetrically – left side of the block is with very low density (26-28kg/m3) compared with the right side 30-32kg/m3). Compression stress is positively correlated with probe density and the compression stress distributions resemble the distribution of density. Apparently, larger amount of small particles of the raw material C are jamming steam flow to the central parts of block form, causing inconsistency of temperature field, and resulting in lower inter-particle melting degree. Considering investigated raw materials preference must be given to material B