A graph-theory formalism of the organic chemistry is suggested. The molecular system is considered as a multigraph with loops, the vertices are evaluated by their mapping onto the vocabulary of vertex labels (e.g. atomic symbols). A multiedge of multiplicity t corresponds to a t-tuple (single, double, triple, etc) bond. The chemical reaction of molecular systems is treated by the transformation of graphs. The suggested graph-theory approach allows to formalize many notions and concepts that are naturally emerging in the computer simulation of organic chemistry.