The irradiation stability of polystyrene (PS) was studied by ¹³C and ¹H NMR spectra, Nuclear Overhauser Relaxation (NOE) and ¹³C NMR spin-lattice relaxation time (T_l). The results indicate that ¹³C and ¹H NMR chemical shifts, NOE and T_l were almost invariant with the increase of irradiation dose. This shows that polystyrene is particularly stable within 2.5 kGy doses and the mechanism of its stability is discussed.