  A pebbling move on a graph $G$ consists of taking two pebbles off one vertex
and placing one on an adjacent vertex. The pebbling number of a graph $G$,
denoted by $f(G)$, is the least integer $n$ such that, however $n$ pebbles are
located on the vertices of $G$, we can move one pebble to any vertex by a
sequence of pebbling moves. Let $M(G)$ be the middle graph of $G$. For any
connected graphs $G$ and $H$, Graham conjectured that $f(G\times H)\leq
f(G)f(H)$. In this paper, we give the pebbling number of some graphs and prove
that Graham's conjecture holds for the middle graphs of some even cycles.
