Is there any graph that all its vertices are cut vertices? I couldn't find a graph with this property? and if there is no such graph how can i prove that it does not exist.
There is no such (finite) graph.
Suppose such a graph $G$ existed. We may assume WLOG it is connected. For each vertex $v$, let $f(v)$ be the minimum number of vertices in the connected components of the graph $G - v$ obtained by deleting $v$. Take $v$ that minimizes $f$, and let $C$ be a connected component of $G - v$ with $f(v)$ vertices. At least one vertex $w$ of $C$ is adjacent to $v$. Now in $G - w$, one connected component must contain $v$, and this will also contain all other vertices not in $C$. Any other connected component of $G - w$ will therefore be a proper subset of $C$, so $f(w) < f(v)$, contradiction.