Four kinds of annealed polyethylene (PE) films with varying densities were photo-irradiated with a medium pressure mercury lamp. Photodegradation processes were followed by ultraviolet (UV) and Fourier transform infrared (FTIR) spectroscopic techniques and by gel and mechanical property measurements. Oxygenated products build-up was more favored in linear low density (LLD) PE and the rate of formation increased in the reverse order of polymer density. A similar trend was seen in gel formation. Measurements of elongation at break (%) showed that the embrittlement time of photo-irradiated PE increased with the decrease of polymer density. The experimental results on unquenched samples were compared with those of quenched samples. It was found that the density (or crystallinity) and crystal size of PE play important roles in determining photostability.