The paper scoped to identify specific energy demand of knife milled beech chips in dependence on moisture, initial and final particle size. The experiments were conducted for beech chips of moisture being 0.5, 7.5 and 16 % wt. dry mass, under mechanical size reduction in the knife mill with the fixed rotational speed of single blade rotor. The specific energy demand of 8.1 - 55.6 kWh t-1 was determined to reach final particle size 0.3 - 1.0 mm from initial 3.4, all as D50 value for given moistures. Regarding the brittle behaviour of beech chips, the Rittinger theory was applied, and the empiric model was defined to predict the energy requirement of size reduction relation to moisture and biomass particle size.