In this paper, using the method of moving planes, we study the monotonicity in some directions and symmetry of the Dirichlet problem involving the fractional Laplacian
−
Δ
α
/
2
u
x
=
f
u
x
,
x
∈
Ω
,
u
x
>
0
,
x
∈
Ω
,
u
x
=
0
,
x
∈
ℝ
n
\
Ω
,
in a slab-like domain
Ω
=
ℝ
n
−
1
×
0
,
h
⊂
ℝ
n
.