  The number of the positive, negative and zero eigenvalues in the spectrum of
the (edge)-weighted graph $G$ are called positive inertia index, negative
inertia index and nullity of the weighted graph $G$, and denoted by $i_+(G)$,
$i_-(G)$, $i_0(G)$, respectively. In this paper, the positive and negative
inertia index of weighted trees, weighted unicyclic graphs and weighted
bicyclic graphs are discussed, the methods of calculating them are obtained.
