The purpose of the research was to determine whether the diameter and defects of wood affect the efficiency of production and fuel consumption. Measurements for the preparation of the bachelor’s thesis were performed on the private property at Brezje pri Tržiču. The influence of knots and diameter of beech assortments was ascertained on productivity and the consumption of fuel in the production of firewood. The assortments were classified into groups on the basis of the list of errors. Five assortments were approximately 50 cm in diameter. Fifteen assortments were approximately 20 cm in diameter. The assortments were classified to quality classes by means of the appropriate standard. The consumption of fuel was measured in tractors and circular saws. A time study of firewood production and a cost calculation of the used machines were made. Various efficiencies of the production between thinner and thicker assortiments were proved by means of statistical methods. In the case of the operation splitting, the efficiency of the production of thinner assortments was 1 m3/h higher as in the case of thicker assortiments. In circular saw sawing of thicker assortiments the efficiency was higher for 7.8 m3/h in comparison to the thinner assortiments. Knots also influence the efficiency of firewood production. The efficiency of the production from the assortiments with knots over 40 mm was 0.27 m3/h lower on the level of the cycle, and, in the operation splitting, 1.41 m3/h in comparison to the assortiments without knots. The average consumption of fuel of the circular amounted to 104 ml/m3. The average consumption of lubricant was 23 ml/m3. The consumption of fuel per unit of the product was ascertained higher (for 33 ml/m3) in thinner assortments. The consumption of fuel per unit of the product was ascertained for 40 ml/m3 higher in assortments with knots. The price of labor was on average 17.99 €/m3. The price of labor was 15.59 €/m3 in thinner assortments. The price of labor was 18.91 €/m3 in thicker assortments.