The paper presents the effect of moisture on the particle size distribution of knife-milled beech chips. The beech chips of 0.5, 7.5, 15.9 and 30.9 wt % in moisture were milled by a knife mill equipped with a single-bladed rotor under the constant peripheral speed of revolution of 10.2 m·s-1 (1,500 min-1). It was found that the RRSB model precisely fits cumulative mass particle size distribution. The Rosin–Rammler size parameter 0.412 - 4.240 mm varies concerning biomass moisture and size of drum screen sieve. Assuming empiric modelling approaches, it was found that the Rosin–Rammler particle size exponentially decreases with a decrease in knife mill screen sizes and linearly increases with biomass moisture. The polydispersity index was independent of wood chips moisture and knife mill screen size, evincing the value of 2.224 ± 0.328.