This paper discusses the concepts about the fault tree’s dual tree and success tree as follows: 1) The dual tree and the success tree are two different concepts. 2) If the fault tree’s mathematical expression is （X）

then its dual tree’s mathematical expression will be defined as D（X） = 1 - （1 - X）

the relationship between the fault tree and its dual tree being that their "gates" are different

"AND" "OR"

and their "bottom events" are completely identical. At the same time

the cut set of the dual tree is the path set of the faulttree and vice versa. 3) The success tree’s mathematical expression is （X） = 1 - （X）

the relationship between the fault tree and its success tree being that their "gates" are different

"AND" "OR"

and their "bottom events" are also different and complementary. Strictly speaking

the cut set of the success tree is not the path set of the fault tree and vice versa.