AbstractThe initiation and growth of the electrical tree in crystalline polymers are influenced by the superstructure. However, there are still many unclear points in the relationship between the tree deterioration and the superstructure (spherulites).To study the relationship between the tree deterioration and the spherulite, we designed the measuring apparatus which allows simultaneous observation of these phenomena. The sample was set on the stage of the polarizing microscope to make observations of the tree initiation and growth in polypropylene.This paper reports the results of some observations concerning the tree initiation and growth in polypropylene with 60‐Hz ac voltage. The results are as follows:1Prior to the initiation of trees, a fine deterioration zone is formed on spherulite surfaces. Thereafter, the discharge occurs along the spherulite surface, and the trees occur.2Tree shapes change under the influence of the distribution state of spherulite.3Tree initiation voltage is 5.8 kV for the spherulite inside and 5.1 kV for the spherulite boundary, respectivel