The modification of powdered ultradisperse cellulose by hindered phenol fragments was carried out. The modification was carried out by reacting cellulose tosylate with 3-(3',5'-ditert-butyl-4'-hydroxyphenyl) propionic acid hydrazide and 3,5-ditert-butyl-4-hydroxybenzyl dimethylamine in DMF at 100 ○C for 16–30 hours. Samples of modified cellulose were characterized by spectroscopy IR and 1H NMR. From the of elemental analysis data, the degree of substitution of cellulose derivatives by hindered phenol units was calculated. The antiradical activity of the obtained samples in their reactions with a stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was determined. The reactions of modified cellulose with DPPH were carried out under the pseudo first-order conditions with respect to the radical. From the values of the effective constants, the second-order rate constants were calculated. It was established that the modification of cellulose by hindered phenol fragments leads to a sharp increase in its antiradical activity, which depends on the degree of substitution of the sample and the method of substitution. The difference in the activity of hydrazide and benzyl derivatives of cellulose may indicate their different spatial structure, resulting in different availability of phenol fragments. The antiradical activity of the hydrazide derivative of cellulose exceeds that of the antioxidant Ionol (2,6-ditert-butyl-4-methylphenol).