The sensitivity of the photosynthetic apparatus to ultraviolet‐B (UV‐B) irradiation was studied in cultures of unicellular green alga Scenedesmus obliquus incubated in low light (low photosynthetically active radiation intensity [LL]) and high light (high photosynthetically active radiation intensity [HL]) conditions, treated or not with exogenous polyamines. Biochemical and physicochemical measurement showed that UV‐B radiation induces a decrease in the thylakoid‐associated putrescine (Put) and an increase in spermine (Spm), so that the reduction of Put/Spm ratio leads to the increase of light‐harvesting complex II (LHCII) size per active reaction center and, consequently, the amplification of UV‐B effects of the photosynthetic apparatus. The separation of oligomeric and monomeric froms of LHCII from isolated thylakoids showed that UV‐B induces an increase in the oligomeric forms of LHCII, which was more intense in LL than in HL. By manipulating the LHCII size with exogenous polyamines, the sensitivity degree of the photosynthetic apparatus to UV‐B changed significantly. Specifically, the addition of Put decreased highly the sensitivity of LL culture to UV‐B because of the inhibitory effect of Put on the LHCII size increasing, wheres the addition of Spm enhanced the UV‐B injury induced in HL culture because of the increasing of LHCII size. The ability of the photosynthetic apparatus to recover the UV‐B induced changes was also investigated.