  For a simple signed graph $G$ with the adjacency matrix $A$ and net degree
matrix $D^{\pm}$, the net Laplacian matrix is $L^{\pm}=D^{\pm}-A$. We introduce
a new oriented incidence matrix $N^{\pm}$ which can keep track of the sign as
well as the orientation of each edge of $G$. Also $L^{\pm}=N^{\pm}(N^{\pm})^T$.
Using this decomposition, we find the numbers of positive and negative spanning
trees of $G$ in terms of the principal minors of $L^{\pm}$ generalizing Matrix
Tree Theorem for an unsigned graph. We present similar results for the signless
net Laplacian matrix $Q^{\pm}=D^{\pm}+A$ along with a combinatorial formula for
its determinant.
