It is well known that electrical trees are greatly influenced by the morphological structure in crystalline polymers. In this paper, samples of polyethylene films, including low-density polyethylene, linear low-density polyethylene and high-density polyethylene have been crystallized and re-crystallized by annealing. The tree initiation voltages of these samples have been measured. It was found that after re-crystallizing the crystallinity and thickness of lamella increase, while the size of the spherulites hardly changes. It was also found that the tree initiation voltage increases after re-crystallizing. Based on the concept of mean free path in solid dielectric, it was assumed that the increase of tree initiation voltage was inherently related to the decrease of mean free path in the spherulites that was due to the increase of the thickness of lamellae.