  In this paper we discuss thermodynamics of apparent horizon of an
$n$-dimensional Friedmann-Robertson-Walker (FRW) universe embedded in an
$(n+1)$-dimensional AdS spacetime. By using the method of unified first law, we
give the explicit entropy expression of the apparent horizon of the FRW
universe. In the large horizon radius limit, this entropy reduces to the
$n$-dimensional area formula, while in the small horizon radius limit, it obeys
the $(n+1)$-dimensional area formula. We also discuss the corresponding bulk
geometry and study the apparent horizon extended into the bulk. We calculate
the entropy of this apparent horizon by using the area formula of the
$(n+1)$-dimensional bulk. It turns out that both methods give the same result
for the apparent horizon entropy. In addition, we show that the Friedmann
equation on the brane can be rewritten to a form of the first law, $dE=TdS
+WdV$, at the apparent horizon.
