A perfect Roman dominating function on a graph
G
is a function
f
:
V
G
⟶
0,1,2
for which every vertex
v
with
f
v
=
0
is adjacent to exactly one neighbor
u
with
f
u
=
2
. The weight of
f
is the sum of the weights of the vertices. The perfect Roman domination number of a graph
G
, denoted by
γ
R
p
G
, is the minimum weight of a perfect Roman dominating function on
G
. In this paper, we prove that if
G
is the Cartesian product of a path
P
r
and a path
P
s
, a path
P
r
and a cycle
C
s
, or a cycle
C
r
and a cycle
C
s
, where
r
,
s
>
5
, then
γ
R
p
G
≤
2
/
3
G
.