A watching system in a graph G=(V, E) is a set W={w1, w2, …,wk}, where wi=(vi, Zi); viÎ V and Zi is a subset of closed neighborhood of vi such that the sets LW(v) ={wi: vÎZi} are non-empty and distinct, for any v ÎV. In this paper, we study the watching systems of line graph Kn which is called Triangular graph and denoted by T (n). The minimum size of a watching system of G is denoted by w (G). We show that w (T(n))= [2n/3].