  We study the relationship between $n$-cluster tilting modules over $n$
representation finite algebras and the Euler forms. We show that the dimension
vectors of cluster-indecomposable modules give the roots of the Euler form.
Moreover, we show that cluster-indecomposable modules are uniquely determined
by their dimension vectors. This is a generalization of Gabriel's theorem by
cluster tilting theory. We call the above roots cluster-roots and investigate
their properties. Furthermore, we provide the description of quivers with
relations of $n$-APR tilts. Using this, we provide a generalization of BGP
reflection functors.
