Carbon nanoparticles attract attention of plant researchers as a possible means of improving crop yield and its quality. There are grounds to believe that the beneficial influence of polyhydroxy fullerene (PHF) on plants is due to its antioxidant activity, but the mechanism of its action on their growth and development remains unclear. Our study shows that PHF added to the nutrient medium accelerates barley roots elongation owing to the increase of their longitudinal extensibility in the growth zone. The impact of PHF on root growth was much more pronounced under the action of stressors inducing the accumulation of reactive oxygen species, such as UV-B radiation, salt stress and the excess of salicylic acid. Dichlorofluorescein assay showed that PHF prevented oxidative stress development and subapical root swelling after UV-B irradiation of roots. The conclusion is drawn that the important reason of root growth acceleration in the presence of PHF is its ability to serve as a scavenger of free radicals. That’s why it may be especially useful for the improvement of plant growth under environmental stress.