Abstract: In this paper, bubble nucleation was studied during free foaming of polyethylene. Theoretical deduction on the effect of foaming agent concentraton and temperature on bubble nucleation was made based on classical nucleation theory and reasonable hypothesis. The deduced result basically agreed with the experimental one, which means that the classical nucleation theory can describe well the process of free foaming. The results indicate that the nucleation rate increases with increasing the foaming agent concentration, and the curve is depressed. As the temperature increased, the nucleation rate increased, reached a maximum and remained, finally decreased with further increasing the temperature.