The icing of transmission lines seriously threatens the safety of power grid. The icing process of transmission lines is complicated and involves many influential factors. The accurate understanding of the law of icing is the basis for the establishment and optimization of an icing numerical model. Different kinds of conductors were tested under natural icing conditions on Xuefeng Mountain Natural Icing Test Base. The effects of conductor diameter, conductor surface treatment, icing type and torsion on icing process were studied. The characteristics of icing on conductors with different diameters were analyzed by calculating the water droplet collision efficiency. The results show that under the natural environment, the wind velocity was the key factor which determined the rime shape on transmission lines. Meanwhile, the icing thickness increased nonlinearly with time. The smaller the conductor diameter, the faster the icing thickness increased. The conductor torsion made the leeward side of conductor turn to the windward side, which increased the icing rate. When the icing type became glaze, in addition to the windward wing icing, more icicles were likely formed under a conductor. As a result, the ice structure became more complex.