All graphs under consideration are finite, simple, connected, and undirected. Adjacency matrix of a graph
G
is 0,1 matrix
A
=
a
i
j
=
0
,
i
f
v
i
=
v
j
o
r
d
v
i
,
v
j
≥
2
1
,
i
f
d
v
i
,
v
j
=
1.
. Here in this paper, we discussed new type of adjacency matrix known by 1-2 adjacency matrix defined as
A
1,2
G
=
a
i
j
=
0
,
i
f
v
i
=
v
j
o
r
d
v
i
,
v
j
≥
3
1
,
i
f
d
v
i
,
v
j
=
2
, from eigenvalues of the graph, we mean eigenvalues of the 1-2 adjacency matrix. Let
T
n
c
be the set of the complement of trees of order
n
. In this paper, we characterized a unique graph whose least eigenvalue is minimal among all the graphs in
T
n
c
.