A new cyclic, hindered amine light stabilizer (HALS) was prepared by the addition polymerization of a diepoxide and 4-amino-2,2,6,6-tetramethylpiperidine, and its photostabilizing effects on styrenic polymers were examined. The chemical structure of cyclic HALS was characterized by MALDI-TOF mass spectrometry. The prepared cyclic HALS had a moderate molecular weight ($overline{M}_w$=900~1,100) and high thermal stability ($T_d{approx}300^{circ}C$) that were compatible with styrenic polymers, such as polystyrene and styrene-butadiene rubber. The photostabilization effect of cyclic HALS was similar to or better than that of Tinuvin 770, a commercially available polymeric HALS. A synergistic effect between cyclic HALS and a UV absorber on the inhibition of photooxidation was observed.