The main goal of this study was to investigate the influence of pH and grain size of biochar on its physicochemical properties and on the quantity and quality of humic substances released from biochar. The studies were conducted on 5 biochar fractions and on the mixture of all fractions. Influence of grain size on physicochemical properties of biochar was characterized by analyzing changes in total carbon content, ash, density, total surface charge, average adsorption energy, specific surface area, content of functional groups and FTIR spectra. The effect of pH and grain size of biochar on releasing organic compounds were examined at pH of the extraction solution: 3, 5, 7 and 9 by analyzing aqueous extracts for changes in organic carbon content and for variation of humification degree coefficients. The results showed the grain size affected physicochemical properties of investigated material. The fraction < 0.5 mm was characterized by the highest density, ash, content of functional groups and the lowest carbon content. Increasing grain diameter corresponded with decrease in average adsorption energy and with increase in the specific surface area. Only grain size, not pH of the solution, revealed clear effect on the quantity and quality of humic substances released from biochar.