The thermogravimetric method was used to study one-stage pyrolysis of finely ground flax shive chemically activated by NH4NO3 (AN) and NH4Cl (NAC). Activated carbons (AC) were obtained and the chemical activation mechanism was determined. The morphology, surface area and pore size distribution of the AC samples were studied by the methods of scanning electron microscopy, low-temperature nitrogen adsorption, energy-dispersive X-ray spectroscopy and X-ray diffraction. The maximum methylene blue adsorption capacity of the obtained AC samples was determined. The optimal parameters of activated flax shive pyrolysis (temperature, time) required for obtaining AC with the maximum specific surface area and adsorption capacity were identified.