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Although electrical trees in silicon rubbers, silicone rubbers have been observed, AC trees have many small branches and thus it is hard to measure it in 3D. In order to observe the patterns of AC trees, two-dimensional sample is used. The development mode of the tree has been classified into three modes; Just after generating the tree, (i) the AC tree has developed straightly. Then the branches has started and (ii) the envelope of the top of the branches has developed coaxially. When one of the branches reached at some point, (iii) that branch developed faster than others and then breakdown occurred. It is known that trees disappear in silicon rubbers after some hours. Trees in twoor three-dimensional silicon rubbers have been compared. Small branches has disappeared after 24 hours in 3-D sample. In 2-D samples, trees did not disappear as ones in 3-D samples.