In this paper. a graph theoretic analog to the celebrated Faber-Krahn inequ
ality for the first eigenvalue of the Dirichlet problem of the Laplacian Fo
r a bounded domain in the Euclidean space is shown. Namely, the optimal est
imate of the first eigenvalue of the Dirichlet boundary problem of the comb
inatorial Laplacian for a graph with boundary is given.